diff --git a/.env.example b/.env.example
index a7ef3e7..795bbf2 100644
--- a/.env.example
+++ b/.env.example
@@ -59,4 +59,7 @@ VISIT_REQUEST_TTL_MINUTES=35
 # 每窗请求数上限——去重本身为主缓解）与 ov 互动流水清理调度间隔（分钟，0=停用；
 # 清理 like_batches/visit_requests/visit_sessions 超窗口行，累计总数永不清）。
 VISIT_WINDOW_REQUESTS=60
+# P4a Task 4：公开照片下载端点基础限速（来源 IP 固定窗，窗口=60s 固定；
+# 每窗请求数上限——签名现算无幂等面，限速定位是误客户端缓冲，P5 速率层接防）。
+DOWNLOAD_WINDOW_REQUESTS=120
 CLEANUP_INTERVAL_MINUTES=60
diff --git a/apps/viewer/e2e/download.spec.ts b/apps/viewer/e2e/download.spec.ts
new file mode 100644
index 0000000..c405387
--- /dev/null
+++ b/apps/viewer/e2e/download.spec.ts
@@ -0,0 +1,130 @@
+import { expect, test, type Page } from '@playwright/test';
+import {
+  cleanupSeededAlbums,
+  flushSync,
+  seedAlbum,
+  seedPhoto,
+} from './helpers';
+
+// P4a Task 4 下载 Playwright（真实后端 + 真实签名管线）：
+//   · 入口可见性随相册两级开关（off/off 无按钮 / on/off 单按钮 / on/on 分体
+//     菜单两选项）；
+//   · 微信内置浏览器 UA mock（addInitScript 覆写 navigator.userAgent）→ 点击
+//     下载出引导浮层（不触端点、不 window.open），「知道了」收起；
+//   · 真实下载点击：download 端点 200 + window.open 收到签名 URL（对象键模式
+//     断言；实际文件保存不在浏览器 e2e 域——由 api e2e 字节级覆盖）。
+
+/** 种「下载相册」：1 张已发布照片，开关组合由参数给定。 */
+async function seedDownloadAlbum(
+  alias: string,
+  switches: { downloadEnabled?: boolean; originalDownloadEnabled?: boolean },
+): Promise<string> {
+  const album = await seedAlbum({
+    alias,
+    translations: [{ locale: 'zh-CN', title: `下载测试相册 ${alias}` }],
+    ...switches,
+  });
+  await seedPhoto(album.id, `${alias}-p0`, {
+    width: 480,
+    height: 300,
+    publishedAt: new Date(Date.now() - 60_000),
+  });
+  await flushSync(album.id);
+  return album.publicToken;
+}
+
+async function openLightbox(page: Page, token: string, init?: () => void): Promise<void> {
+  await page.addInitScript(() => localStorage.setItem('viewer.locale', 'zh-CN'));
+  if (init) await page.addInitScript(init);
+  await page.goto(`/a/${token}`);
+  await expect(page.locator('.cell').first()).toBeVisible();
+  await page.locator('.cell').first().click();
+  await expect(page.locator('.lightbox')).toBeVisible();
+}
+
+test.afterAll(async () => {
+  await cleanupSeededAlbums();
+});
+
+test('download entry visibility follows album two-tier switches', async ({ page }) => {
+  test.setTimeout(120_000);
+  // off/off：无下载入口。
+  const offOff = await seedDownloadAlbum('dl-offoff', {
+    downloadEnabled: false,
+    originalDownloadEnabled: false,
+  });
+  await openLightbox(page, offOff);
+  await expect(page.locator('.download-group')).toHaveCount(0);
+
+  // on/off：单按钮（无 caret/菜单）。
+  const onOff = await seedDownloadAlbum('dl-onoff', {});
+  await openLightbox(page, onOff);
+  await expect(page.locator('.control.download')).toBeVisible();
+  await expect(page.locator('.control.download-caret')).toHaveCount(0);
+
+  // on/on：分体按钮，菜单两选项（下载图片 / 下载原图）。
+  const onOn = await seedDownloadAlbum('dl-onon', { originalDownloadEnabled: true });
+  await openLightbox(page, onOn);
+  await expect(page.locator('.control.download-caret')).toBeVisible();
+  await page.locator('.control.download-caret').click();
+  await expect(page.locator('.download-menu button')).toHaveText(['下载图片', '下载原图']);
+});
+
+test('WeChat webview UA shows guidance overlay instead of downloading', async ({ page }) => {
+  test.setTimeout(120_000);
+  const token = await seedDownloadAlbum('dl-wechat', {});
+  // UA 覆写先于应用代码（addInitScript）；MicroMessenger 触发引导浮层面。
+  await openLightbox(page, token, () => {
+    const ua =
+      'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148 MicroMessenger/8.0.49(0x18003133) NetType/WIFI Language/zh_CN';
+    Object.defineProperty(window.navigator, 'userAgent', { get: () => ua, configurable: true });
+  });
+
+  let endpointHit = false;
+  page.on('request', (request) => {
+    if (request.url().includes('/download?')) endpointHit = true;
+  });
+
+  await page.locator('.control.download').click();
+  await expect(page.locator('.wechat-guide[role="dialog"]')).toBeVisible();
+  await expect(page.locator('.wechat-guide h2')).toHaveText('保存图片指引');
+  await expect(page.locator('.guide-copy')).toBeVisible();
+
+  // 「知道了」收起浮层；期间 download 端点零命中（引导替代直接下载）。
+  await page.locator('.guide-dismiss').click();
+  await expect(page.locator('.wechat-guide')).toHaveCount(0);
+  expect(endpointHit).toBe(false);
+});
+
+test('real click downloads browse via signed url from the endpoint', async ({ page }) => {
+  test.setTimeout(120_000);
+  const token = await seedDownloadAlbum('dl-real', {});
+  // 捕获 window.open（避免真实弹出标签页）：记录 URL 供对象键断言。
+  await openLightbox(page, token, () => {
+    (window as unknown as { __openedUrls: string[] }).__openedUrls = [];
+    window.open = ((url: string | URL) => {
+      (window as unknown as { __openedUrls: string[] }).__openedUrls.push(String(url));
+      return {} as Window;
+    }) as typeof window.open;
+  });
+
+  const endpoint = page.waitForResponse(
+    (response) =>
+      response.url().includes('/download?variant=browse') && response.status() === 200,
+  );
+  await page.locator('.control.download').click();
+  const response = await endpoint;
+  expect((await response.json()).variant).toBe('browse');
+
+  await expect
+    .poll(async () =>
+      page.evaluate(() => (window as unknown as { __openedUrls: string[] }).__openedUrls[0] ?? ''),
+    )
+    .toContain('/variants/');
+  const opened = await page.evaluate(
+    () => (window as unknown as { __openedUrls: string[] }).__openedUrls[0] ?? '',
+  );
+  // 短期签名凭证（SigV4 查询参数面）+ browse 对象键（无水印相册）。
+  expect(opened).toContain('X-Amz-Signature');
+  expect(opened).toContain('/browse.jpg');
+});
diff --git a/apps/viewer/e2e/helpers.ts b/apps/viewer/e2e/helpers.ts
index ca63fc5..3a69a52 100644
--- a/apps/viewer/e2e/helpers.ts
+++ b/apps/viewer/e2e/helpers.ts
@@ -53,6 +53,9 @@ export interface SeedAlbumOptions {
   }>;
   status?: string;
   publicEnabled?: boolean;
+  /** 下载两级开关（P4a Task 4）：缺省库默认 true/false（同 D06 工作默认）。 */
+  downloadEnabled?: boolean;
+  originalDownloadEnabled?: boolean;
   timezone?: string;
   startAt?: Date;
   endAt?: Date | null;
@@ -72,6 +75,8 @@ export async function seedAlbum(options: SeedAlbumOptions): Promise<{ id: string
       status: options.status ?? 'active',
       defaultLocale: options.defaultLocale ?? 'zh-CN',
       publicEnabled: options.publicEnabled ?? true,
+      downloadEnabled: options.downloadEnabled ?? true,
+      originalDownloadEnabled: options.originalDownloadEnabled ?? false,
     },
     select: { id: true, publicToken: true },
   });
diff --git a/apps/viewer/src/__tests__/download.spec.ts b/apps/viewer/src/__tests__/download.spec.ts
new file mode 100644
index 0000000..e523bb0
--- /dev/null
+++ b/apps/viewer/src/__tests__/download.spec.ts
@@ -0,0 +1,283 @@
+import { flushPromises, mount, type VueWrapper } from '@vue/test-utils';
+import { defineComponent, ref } from 'vue';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import {
+  getPhotoDownload,
+  getPublicPhotos,
+  type PublicAlbum,
+  type PublicPhoto,
+  type PublicPhotoDownload,
+  type PublicPhotoList,
+} from '../api/client';
+import { createViewerI18n, type ViewerI18n } from '../i18n';
+import PhotoLightbox from '../components/PhotoLightbox.vue';
+import { usePhotoSequence } from '../feed/sequence';
+import {
+  isExpired,
+  isWechatWebview,
+  useDownload,
+  type DownloadController,
+} from '../download/useDownload';
+import { makeAlbum } from './fixtures';
+
+// P4a Task 4 下载（T12 收口）：开关→入口可见性矩阵、分体菜单选项、微信 UA
+// 判定与引导浮层、过期凭证重取一次、失败提示、复制深链。
+
+vi.mock('../api/client', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../api/client')>();
+  return {
+    ...actual,
+    getPublicPhotos: vi.fn(),
+    getPublicPhoto: vi.fn(),
+    getPhotoDownload: vi.fn(),
+  };
+});
+
+const mockedGetPublicPhotos = vi.mocked(getPublicPhotos);
+const mockedGetPhotoDownload = vi.mocked(getPhotoDownload);
+
+const WECHAT_IOS_UA =
+  'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148 MicroMessenger/8.0.49(0x18003133) NetType/WIFI Language/zh_CN';
+const WECHAT_ANDROID_UA =
+  'Mozilla/5.0 (Linux; Android 14; Pixel 8) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/122.0.0.0 Mobile Safari/537.36 MicroMessenger/8.0.49.2500(0x28004935)';
+const WECHAT_WINDOWS_UA =
+  'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/107.0.0.0 Safari/537.36 MicroMessenger/7.0.20.1781(0x670c0123) WindowsWechat(0x63090a13)';
+const PLAIN_IOS_SAFARI_UA =
+  'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1';
+const DESKTOP_CHROME_UA =
+  'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36';
+
+function makePhoto(id: string): PublicPhoto {
+  return {
+    photoId: id,
+    width: 800,
+    height: 500,
+    thumb: { url: `http://img.test/${id}-thumb.jpg?s=1`, expiresAt: '2026-10-01T00:00:00.000Z' },
+    browse: { url: `http://img.test/${id}-browse.jpg?s=1`, expiresAt: '2026-10-01T00:00:00.000Z' },
+    likeCount: 0,
+    publishedAt: '2026-10-01T00:00:00.000Z',
+  };
+}
+
+function makeList(items: PublicPhoto[]): PublicPhotoList {
+  return { items, nextCursor: null, hasMore: false };
+}
+
+function credential(url: string, expiresAt: string, variant: 'browse' | 'original'): PublicPhotoDownload {
+  return { url, expiresAt, variant };
+}
+
+const FUTURE = new Date(Date.now() + 14 * 60_000).toISOString();
+const PAST = new Date(Date.now() - 60_000).toISOString();
+
+function stubUserAgent(ua: string): void {
+  Object.defineProperty(window.navigator, 'userAgent', { value: ua, configurable: true });
+}
+
+function restoreUserAgent(): void {
+  // 删除实例自有属性，回落原型 getter（jsdom 原始 UA）。
+  delete (window.navigator as { userAgent?: string }).userAgent;
+}
+
+let i18n: ViewerI18n;
+const mounted: VueWrapper[] = [];
+
+/** window.open 桩（记录调用即可）：经工厂推导 spy 类型，绕开 spyOn 泛型约束。 */
+function spyWindowOpen() {
+  return vi.spyOn(window, 'open').mockReturnValue({} as Window);
+}
+let openSpy: ReturnType<typeof spyWindowOpen>;
+
+/** 宿主实例形态：setup 暴露 albumRef（ref 经代理解包）+ download 控制器。 */
+interface HostVm {
+  albumRef: PublicAlbum | null;
+  download: DownloadController;
+}
+
+/** 宿主：sequence + 相册 ref + 下载控制器 + lightbox（镜像 ActivityPage 组装）。 */
+function mountLightbox(album: PublicAlbum): VueWrapper & { vm: HostVm } {
+  const Host = defineComponent({
+    components: { PhotoLightbox },
+    setup() {
+      const sequence = usePhotoSequence({
+        token: ref('tok-dl'),
+        locale: ref('zh-CN'),
+        category: ref(null),
+      });
+      const albumRef = ref<PublicAlbum | null>(album);
+      const current = ref('p1');
+      const download = useDownload({
+        token: ref('tok-dl'),
+        album: () => albumRef.value,
+        photoId: () => current.value,
+      });
+      return { sequence, photoId: current, download, albumRef };
+    },
+    template: `<PhotoLightbox v-model:photo-id="photoId" :sequence="sequence" :download="download" />`,
+  });
+  const wrapper = mount(Host, { global: { plugins: [i18n] } });
+  mounted.push(wrapper);
+  return wrapper as VueWrapper & { vm: HostVm };
+}
+
+describe('photo download (P4a Task 4)', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    i18n = createViewerI18n('zh-CN');
+    mockedGetPublicPhotos.mockReset().mockResolvedValue(makeList([makePhoto('p1'), makePhoto('p2')]));
+    mockedGetPhotoDownload.mockReset();
+    openSpy = spyWindowOpen();
+  });
+
+  afterEach(() => {
+    while (mounted.length > 0) {
+      mounted.pop()?.unmount();
+    }
+    restoreUserAgent();
+    vi.restoreAllMocks();
+  });
+
+  it('detects WeChat webview UAs (iOS/Android/Windows) and passes others', () => {
+    for (const ua of [WECHAT_IOS_UA, WECHAT_ANDROID_UA, WECHAT_WINDOWS_UA]) {
+      expect(isWechatWebview(ua)).toBe(true);
+    }
+    for (const ua of [PLAIN_IOS_SAFARI_UA, DESKTOP_CHROME_UA, '']) {
+      expect(isWechatWebview(ua)).toBe(false);
+    }
+  });
+
+  it('treats expiresAt at-or-before now as expired', () => {
+    expect(isExpired(PAST)).toBe(true);
+    expect(isExpired(new Date().toISOString())).toBe(true);
+    expect(isExpired(FUTURE)).toBe(false);
+    expect(isExpired('not-a-date')).toBe(false); // 非法时间戳不误判过期
+  });
+
+  it('controller model: switch matrix incl. off/on cascade (original implies browse)', async () => {
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+    const controller = wrapper.vm.download;
+
+    // on/off（夹具缺省）。
+    expect(controller.canDownload).toBe(true);
+    expect(controller.canDownloadOriginal).toBe(false);
+
+    const cases: Array<[boolean, boolean, boolean, boolean]> = [
+      // [downloadEnabled, originalDownloadEnabled, canDownload, canDownloadOriginal]
+      [true, true, true, true],
+      [true, false, true, false],
+      [false, false, false, false],
+      [false, true, false, false], // 级联：browse 关则 original 项不出现
+    ];
+    for (const [downloadEnabled, originalDownloadEnabled, can, canOriginal] of cases) {
+      wrapper.vm.albumRef = makeAlbum({ downloadEnabled, originalDownloadEnabled });
+      await flushPromises();
+      expect(controller.canDownload).toBe(can);
+      expect(controller.canDownloadOriginal).toBe(canOriginal);
+    }
+  });
+
+  it('toolbar visibility: off/on → no entry; on/off → single button; on/on → split', async () => {
+    // off/off：无下载入口。
+    const off = mountLightbox(makeAlbum({ downloadEnabled: false, originalDownloadEnabled: false }));
+    await flushPromises();
+    expect(off.find('.download-group').exists()).toBe(false);
+
+    // on/off：单按钮，无 caret/菜单。
+    const browseOnly = mountLightbox(makeAlbum());
+    await flushPromises();
+    expect(browseOnly.find('.control.download').exists()).toBe(true);
+    expect(browseOnly.find('.control.download-caret').exists()).toBe(false);
+
+    // on/on：分体按钮（caret 出现，菜单默认收起）。
+    const split = mountLightbox(makeAlbum({ originalDownloadEnabled: true }));
+    await flushPromises();
+    expect(split.find('.control.download-caret').exists()).toBe(true);
+    expect(split.find('.download-menu').exists()).toBe(false);
+    // 主按钮 aria 展开 = false。
+    expect(split.find('.control.download-caret').attributes('aria-expanded')).toBe('false');
+  });
+
+  it('split menu offers both variants; picking original requests the original endpoint', async () => {
+    mockedGetPhotoDownload.mockResolvedValue(credential('http://img.test/p1-original.jpg', FUTURE, 'original'));
+    const wrapper = mountLightbox(makeAlbum({ originalDownloadEnabled: true }));
+    await flushPromises();
+
+    await wrapper.find('.control.download-caret').trigger('click');
+    const items = wrapper.findAll('.download-menu button');
+    expect(items.map((item) => item.text())).toEqual(['下载图片', '下载原图']);
+
+    await items[1]!.trigger('click');
+    await flushPromises();
+    expect(mockedGetPhotoDownload).toHaveBeenCalledWith('tok-dl', 'p1', 'original');
+    expect(openSpy).toHaveBeenCalledWith('http://img.test/p1-original.jpg', '_blank', 'noopener,noreferrer');
+    // 选择后菜单收起。
+    expect(wrapper.find('.download-menu').exists()).toBe(false);
+  });
+
+  it('main button downloads browse; expired credential is re-requested exactly once', async () => {
+    // 首响应已过期（缓存/时钟漂移）→ 重取一次 → 打开第二个 URL。
+    mockedGetPhotoDownload
+      .mockResolvedValueOnce(credential('http://img.test/p1-browse-stale.jpg', PAST, 'browse'))
+      .mockResolvedValueOnce(credential('http://img.test/p1-browse-fresh.jpg', FUTURE, 'browse'));
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.control.download').trigger('click');
+    await flushPromises();
+    expect(mockedGetPhotoDownload).toHaveBeenCalledTimes(2);
+    expect(mockedGetPhotoDownload).toHaveBeenCalledWith('tok-dl', 'p1', 'browse');
+    expect(openSpy).toHaveBeenCalledWith('http://img.test/p1-browse-fresh.jpg', '_blank', 'noopener,noreferrer');
+  });
+
+  it('fresh credential opens directly without a second request; failure shows a retryable notice', async () => {
+    mockedGetPhotoDownload.mockResolvedValue(credential('http://img.test/p1-browse.jpg', FUTURE, 'browse'));
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.control.download').trigger('click');
+    await flushPromises();
+    expect(mockedGetPhotoDownload).toHaveBeenCalledTimes(1);
+    expect(openSpy).toHaveBeenCalledWith('http://img.test/p1-browse.jpg', '_blank', 'noopener,noreferrer');
+    expect(wrapper.find('.download-failed').exists()).toBe(false);
+
+    // 端点失败（网络/403 等）→ 失败提示可再见（再次点击重试）。
+    openSpy.mockClear();
+    mockedGetPhotoDownload.mockReset().mockRejectedValue(new Error('boom'));
+    await wrapper.find('.control.download').trigger('click');
+    await flushPromises();
+    expect(wrapper.find('.download-failed[role="alert"]').text()).toContain('下载失败');
+    expect(openSpy).not.toHaveBeenCalled();
+  });
+
+  it('WeChat webview: click opens the guidance overlay instead of hitting the endpoint', async () => {
+    stubUserAgent(WECHAT_IOS_UA);
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.control.download').trigger('click');
+    await flushPromises();
+    expect(wrapper.find('.wechat-guide[role="dialog"]').exists()).toBe(true);
+    expect(wrapper.find('.wechat-guide h2').text()).toBe('保存图片指引');
+    expect(mockedGetPhotoDownload).not.toHaveBeenCalled();
+    expect(openSpy).not.toHaveBeenCalled();
+
+    // 知道了：浮层收起（再点击可重新触发）。
+    await wrapper.find('.guide-dismiss').trigger('click');
+    expect(wrapper.find('.wechat-guide').exists()).toBe(false);
+  });
+
+  it('guidance copy link writes the current photo deep link with feedback', async () => {
+    stubUserAgent(WECHAT_IOS_UA);
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    Object.defineProperty(window.navigator, 'clipboard', { value: { writeText }, configurable: true });
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.control.download').trigger('click');
+    await wrapper.find('.guide-copy').trigger('click');
+    await flushPromises();
+    expect(writeText).toHaveBeenCalledWith(window.location.href);
+    expect(wrapper.find('.copy-status').text()).toBe('已复制链接');
+  });
+});
diff --git a/apps/viewer/src/__tests__/fixtures.ts b/apps/viewer/src/__tests__/fixtures.ts
index d6a29d7..b201a6e 100644
--- a/apps/viewer/src/__tests__/fixtures.ts
+++ b/apps/viewer/src/__tests__/fixtures.ts
@@ -27,6 +27,9 @@ export function makeAlbum(
     ],
     banner: null,
     viewCount: null,
+    // P4a Task 4：下载两级开关（契约 PublicAlbum 必返字段；缺省 on/off）。
+    downloadEnabled: true,
+    originalDownloadEnabled: false,
     ...rest,
   };
 }
diff --git a/apps/viewer/src/api/client.ts b/apps/viewer/src/api/client.ts
index 9907b11..8a116ee 100644
--- a/apps/viewer/src/api/client.ts
+++ b/apps/viewer/src/api/client.ts
@@ -12,6 +12,10 @@ export type LikeBatchResponse = components['schemas']['LikeBatchResponse'];
 export type VisitReportRequest = components['schemas']['VisitReportRequest'];
 export type VisitReportResponse = components['schemas']['VisitReportResponse'];
 
+// P4a Task 4 下载端点类型（downloadPublicPhoto，ov 域；A18 两级开关）。
+export type PublicPhotoDownload = components['schemas']['PublicPhotoDownload'];
+export type DownloadVariant = components['schemas']['DownloadVariant'];
+
 /** 公开照片排序模式（sort=hot 仅 ov 实现：likeTotal 倒序短期快照，§7）。 */
 export type SortMode = 'latest' | 'hot';
 
@@ -146,3 +150,16 @@ export function postAlbumVisit(token: string, body: VisitReportRequest): Promise
     body: JSON.stringify(body),
   });
 }
+
+/** 公开照片下载凭证（P4a Task 4）：短期签名 URL，过期经本端点刷新（重取一次）。 */
+export function getPhotoDownload(
+  token: string,
+  photoId: string,
+  variant: DownloadVariant,
+): Promise<PublicPhotoDownload> {
+  return apiRequest<PublicPhotoDownload>(
+    withQuery(`/public/albums/${encodeURIComponent(token)}/photos/${encodeURIComponent(photoId)}/download`, {
+      variant,
+    }),
+  );
+}
diff --git a/apps/viewer/src/components/PhotoLightbox.vue b/apps/viewer/src/components/PhotoLightbox.vue
index d5e1df4..e8abcce 100644
--- a/apps/viewer/src/components/PhotoLightbox.vue
+++ b/apps/viewer/src/components/PhotoLightbox.vue
@@ -2,6 +2,7 @@
 import { computed, onBeforeUnmount, onMounted, reactive, ref, watch } from 'vue';
 import { useI18n } from 'vue-i18n';
 import type { PhotoSequence } from '../feed/sequence';
+import type { DownloadController } from '../download/useDownload';
 import type { LikesController } from '../likes/useLikes';
 import { MIN_SCALE, panBy, resetZoom, wheelFactor, zoomAt, type ZoomState } from '../viewer/zoom';
 
@@ -20,6 +21,11 @@ import { MIN_SCALE, panBy, resetZoom, wheelFactor, zoomAt, type ZoomState } from
 //   · 双击（鼠标 dblclick / 触控双 tap）：复位缩放；
 //   · 滚轮（无需 ctrl）：以光标为不动点缩放；Esc/关闭按钮/背景点击关闭。
 //
+// 下载（P4a Task 4）：工具栏入口按相册两级开关渲染（控制器 canDownload /
+// canDownloadOriginal；download prop 缺省不渲染——组件单测/降级组装）。原图
+// 可下时为分体按钮（主按钮=浏览图 + ▾ 菜单两选项）；微信内置浏览器由控制器
+// 判定并改走引导浮层（长按保存 + 复制深链）。
+//
 // 关闭回位（滚动锚点）与 URL 同步由 ActivityPage 负责（见 ActivityPage 注释）。
 
 const props = defineProps<{
@@ -27,6 +33,8 @@ const props = defineProps<{
   photoId: string;
   /** 点赞控制器（Task 10）：缺省不渲染点赞位（组件单测/降级组装）。 */
   likes?: LikesController;
+  /** 下载控制器（P4a Task 4）：缺省不渲染下载位（组件单测/降级组装）。 */
+  download?: DownloadController;
 }>();
 
 const emit = defineEmits<{
@@ -263,6 +271,29 @@ function likeCurrent(): void {
   props.likes.like(photo.value.photoId);
 }
 
+// —— 下载（P4a Task 4）：菜单开合随照片切换/关闭复位 ——
+
+const downloadMenuOpen = ref(false);
+
+watch(
+  () => props.photoId,
+  () => {
+    downloadMenuOpen.value = false;
+  },
+);
+
+/** 菜单选项点击：收起菜单后发起对应 variant 下载。 */
+function pickDownload(variant: 'browse' | 'original'): void {
+  downloadMenuOpen.value = false;
+  void props.download?.download(variant);
+}
+
+/** 主按钮：无原图位时直下浏览图；有原图位时也直下浏览图（菜单提供两项）。 */
+function downloadBrowse(): void {
+  downloadMenuOpen.value = false;
+  void props.download?.download('browse');
+}
+
 // —— 签名过期/下线处理 ——
 
 /** 大图加载失败：静默重取单图接口换新签名 URL；404 → 通知父级移除；其他失败可重试。 */
@@ -348,7 +379,40 @@ onBeforeUnmount(() => {
         <span class="like-count">{{ likes.displayFor(photo) }}</span>
       </button>
       <span v-else class="likes">{{ t('viewer.likeCount', { count: photo.likeCount }) }}</span>
-      <!-- P4a 预留：下载 / 分享操作位（P1b 不实现）。 -->
+      <!-- 下载（P4a Task 4）：两级开关驱动——都关无入口；browse 单按钮；
+           original 可下 → 分体按钮（主=浏览图 + ▾ 菜单两项）。 -->
+      <div
+        v-if="download !== undefined && download.canDownload"
+        class="download-group"
+      >
+        <button
+          type="button"
+          class="control download"
+          :disabled="download.busy"
+          :aria-label="t('download.action')"
+          @click="downloadBrowse"
+        >
+          {{ download.busy ? t('download.busy') : t('download.action') }}
+        </button>
+        <button
+          v-if="download.canDownloadOriginal"
+          type="button"
+          class="control download-caret"
+          :aria-label="t('download.options')"
+          :aria-expanded="downloadMenuOpen"
+          @click="downloadMenuOpen = !downloadMenuOpen"
+        >
+          ▾
+        </button>
+        <div v-if="downloadMenuOpen && download.canDownloadOriginal" class="download-menu" role="menu">
+          <button type="button" role="menuitem" @click="pickDownload('browse')">
+            {{ t('download.action') }}
+          </button>
+          <button type="button" role="menuitem" @click="pickDownload('original')">
+            {{ t('download.original') }}
+          </button>
+        </div>
+      </div>
       <button type="button" class="control close" :aria-label="t('viewer.close')" @click="emit('close')">
         ✕
       </button>
@@ -374,6 +438,41 @@ onBeforeUnmount(() => {
     </button>
 
     <p v-if="extending" class="loading">{{ t('album.loading') }}</p>
+
+    <!-- 下载失败轻提示（端点失败/网络；可重点击重试）。 -->
+    <p v-if="download !== undefined && download.failed" class="download-failed" role="alert">
+      {{ t('download.failed') }}
+    </p>
+
+    <!-- 微信内置浏览器引导（P4a Task 4）：替代直接下载——长按保存 + 复制深链。 -->
+    <div
+      v-if="download !== undefined && download.wechatGuide"
+      class="wechat-guide"
+      role="dialog"
+      aria-modal="true"
+      :aria-label="t('download.wechatTitle')"
+    >
+      <div class="wechat-card">
+        <h2>{{ t('download.wechatTitle') }}</h2>
+        <p>{{ t('download.wechatHint') }}</p>
+        <!-- clipboard 不可用降级：展示可选中文本供手动长按复制。 -->
+        <p v-if="download.copyNotice === 'failed'" class="copy-fallback">{{ download.shareUrl() }}</p>
+        <div class="guide-actions">
+          <button type="button" class="guide-copy" @click="download.copyShareLink()">
+            {{ t('download.copyLink') }}
+          </button>
+          <span v-if="download.copyNotice === 'copied'" class="copy-status" role="status">
+            {{ t('download.copied') }}
+          </span>
+          <span v-else-if="download.copyNotice === 'failed'" class="copy-status" role="status">
+            {{ t('download.copyFailed') }}
+          </span>
+          <button type="button" class="guide-dismiss" @click="download.dismissWechatGuide()">
+            {{ t('download.dismiss') }}
+          </button>
+        </div>
+      </div>
+    </div>
   </div>
 </template>
 
@@ -485,6 +584,125 @@ onBeforeUnmount(() => {
   border-radius: 6px;
   padding: 4px 10px;
 }
+/* —— 下载（P4a Task 4）—— */
+.download-group {
+  position: relative;
+  display: inline-flex;
+  align-items: center;
+}
+.control.download {
+  border: 1px solid rgba(249, 250, 251, 0.35);
+  border-radius: 999px 0 0 999px;
+  padding: 4px 12px;
+  font-size: 0.9rem;
+}
+.control.download:disabled {
+  opacity: 0.55;
+  cursor: default;
+}
+.control.download-caret {
+  border: 1px solid rgba(249, 250, 251, 0.35);
+  border-left: none;
+  border-radius: 0 999px 999px 0;
+  padding: 4px 8px;
+  font-size: 0.8rem;
+}
+.download-menu {
+  position: absolute;
+  top: calc(100% + 6px);
+  right: 0;
+  display: grid;
+  min-width: 9em;
+  border-radius: 8px;
+  background: rgba(17, 24, 39, 0.96);
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.4);
+  overflow: hidden;
+  z-index: 2;
+}
+.download-menu button {
+  border: none;
+  background: transparent;
+  color: #f9fafb;
+  text-align: left;
+  padding: 8px 14px;
+  font-size: 0.9rem;
+  cursor: pointer;
+}
+.download-menu button:hover {
+  background: rgba(255, 255, 255, 0.08);
+}
+.download-failed {
+  position: absolute;
+  bottom: 52px;
+  left: 50%;
+  transform: translateX(-50%);
+  margin: 0;
+  border-radius: 8px;
+  background: rgba(185, 28, 28, 0.92);
+  color: #fef2f2;
+  padding: 6px 14px;
+  font-size: 0.875rem;
+}
+/* —— 微信引导浮层 —— */
+.wechat-guide {
+  position: absolute;
+  inset: 0;
+  z-index: 3;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: rgba(17, 24, 39, 0.72);
+  padding: 24px;
+}
+.wechat-card {
+  max-width: 420px;
+  border-radius: 12px;
+  background: #f9fafb;
+  color: #111827;
+  padding: 20px 24px;
+}
+.wechat-card h2 {
+  margin: 0 0 8px;
+  font-size: 1.05rem;
+}
+.wechat-card p {
+  margin: 0 0 12px;
+  font-size: 0.9rem;
+  line-height: 1.6;
+  white-space: pre-line;
+}
+.copy-fallback {
+  user-select: all;
+  word-break: break-all;
+  background: #e5e7eb;
+  border-radius: 6px;
+  padding: 6px 8px;
+}
+.guide-actions {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  flex-wrap: wrap;
+}
+.guide-actions button {
+  border: none;
+  border-radius: 999px;
+  padding: 6px 16px;
+  font-size: 0.9rem;
+  cursor: pointer;
+}
+.guide-copy {
+  background: #1d4ed8;
+  color: #fff;
+}
+.guide-dismiss {
+  background: #e5e7eb;
+  color: #374151;
+}
+.copy-status {
+  font-size: 0.8rem;
+  color: #6b7280;
+}
 .control.nav {
   position: absolute;
   top: 50%;
diff --git a/apps/viewer/src/download/useDownload.ts b/apps/viewer/src/download/useDownload.ts
new file mode 100644
index 0000000..7f2d530
--- /dev/null
+++ b/apps/viewer/src/download/useDownload.ts
@@ -0,0 +1,147 @@
+// apps/viewer/src/download/useDownload.ts —— 大图下载状态机（P4a Task 4，
+// T12 收口）。两级开关（A18）由相册载荷驱动：
+//   · downloadEnabled=false → 工具栏无下载入口（后端同样 403，前端不渲染）；
+//   · browse-only → 单按钮「下载图片」；
+//   · originalDownloadEnabled（且 downloadEnabled）→ 分体按钮 + 菜单两项
+//     （下载图片 / 下载原图——original 是更高级开关，服务端级联校验）。
+//
+// 下载流：点击 → GET download 端点取短期签名凭证 → window.open 新标签直取
+// 对象存储（跨源 fetch+blob 会撞 MinIO presign 的 CORS 面，新标签是本地仿真
+// 与生产同走的稳健路径；`<a download>` 对跨源 URL 的文件名属性被浏览器忽略，
+// 不采用）。过期重取一次：响应 expiresAt 已过期（缓存/时钟漂移）→ 再请求
+// 本端点一次（契约指定的刷新入口），之后无论新旧都打开（服务端时钟偏移的
+// 极端情形交由用户重点击）。
+//
+// 微信内置浏览器（MicroMessenger UA）：不支持程序化下载/新标签导航——以
+// 引导浮层替代直接下载：长按图片保存指引 + 复制当前照片深链（外部浏览器
+// 打开后可正常下载）。clipboard 不可用时降级展示可选中文本供手动长按复制。
+import { computed, onBeforeUnmount, reactive, ref, type Ref } from 'vue';
+import {
+  getPhotoDownload,
+  type DownloadVariant,
+  type PublicAlbum,
+} from '../api/client';
+
+export interface DownloadInputs {
+  token: Ref<string>;
+  /** 当前相册载荷（开关来源；未加载 = 视为关闭）。 */
+  album: () => PublicAlbum | null;
+  /** 当前大图照片（无当前照片时下载为 no-op）。 */
+  photoId: () => string | null;
+}
+
+/** 微信内置浏览器判定（iOS/Android/桌面微信 UA 均含 MicroMessenger）。 */
+export function isWechatWebview(userAgent: string): boolean {
+  return /MicroMessenger/i.test(userAgent);
+}
+
+/** 凭证是否已过期（时钟漂移/缓存响应的防线；≤ now 视为过期）。 */
+export function isExpired(expiresAt: string, now: number = Date.now()): boolean {
+  const timestamp = Date.parse(expiresAt);
+  return Number.isFinite(timestamp) && timestamp <= now;
+}
+
+/**
+ * 打开下载 URL：新标签直达对象存储。弹窗被拦（window.open 返回 null）→
+ * 合成 `<a target=_blank>` 点击兜底（仍在用户手势帧内，多数浏览器放行）；
+ * 导航结果不可观测，best-effort。
+ */
+export function openDownloadUrl(url: string): void {
+  const opened = window.open(url, '_blank', 'noopener,noreferrer');
+  if (opened !== null) return;
+  const anchor = document.createElement('a');
+  anchor.href = url;
+  anchor.target = '_blank';
+  anchor.rel = 'noopener noreferrer';
+  document.body.appendChild(anchor);
+  anchor.click();
+  anchor.remove();
+}
+
+/** 复制反馈停留毫秒（之后自动消失）。 */
+export const COPY_NOTICE_MS = 2500;
+
+export function useDownload(inputs: DownloadInputs) {
+  const busy = ref(false);
+  const failed = ref(false);
+  /** 微信引导浮层可见（点击下载时处于微信内置浏览器）。 */
+  const wechatGuide = ref(false);
+  const copyNotice = ref<'' | 'copied' | 'failed'>('');
+
+  /** 入口可见性：browse 级开关（相册未加载 = 关闭，不渲染入口）。 */
+  const canDownload = computed(() => inputs.album()?.downloadEnabled === true);
+  /** 原图项可见性：两级级联（downloadEnabled && originalDownloadEnabled）。 */
+  const canDownloadOriginal = computed(
+    () =>
+      inputs.album()?.downloadEnabled === true &&
+      inputs.album()?.originalDownloadEnabled === true,
+  );
+
+  /** 当前照片深链（引导浮层复制目标；大图打开期间即当前 URL）。 */
+  function shareUrl(): string {
+    return window.location.href;
+  }
+
+  async function download(variant: DownloadVariant): Promise<void> {
+    if (isWechatWebview(navigator.userAgent)) {
+      wechatGuide.value = true;
+      return;
+    }
+    const photoId = inputs.photoId();
+    if (photoId === null || busy.value) return;
+    busy.value = true;
+    failed.value = false;
+    try {
+      let credential = await getPhotoDownload(inputs.token.value, photoId, variant);
+      if (isExpired(credential.expiresAt)) {
+        // 过期凭证：重取一次（契约刷新入口即本端点），仍过期则照开（极端
+        // 时钟偏移交用户重点击——不做无限重取）。
+        credential = await getPhotoDownload(inputs.token.value, photoId, variant);
+      }
+      openDownloadUrl(credential.url);
+    } catch {
+      failed.value = true;
+    } finally {
+      busy.value = false;
+    }
+  }
+
+  function dismissWechatGuide(): void {
+    wechatGuide.value = false;
+  }
+
+  let copyTimer: ReturnType<typeof setTimeout> | undefined;
+
+  async function copyShareLink(): Promise<void> {
+    try {
+      await navigator.clipboard.writeText(shareUrl());
+      copyNotice.value = 'copied';
+    } catch {
+      copyNotice.value = 'failed';
+    }
+    if (copyTimer !== undefined) clearTimeout(copyTimer);
+    copyTimer = setTimeout(() => {
+      copyNotice.value = '';
+    }, COPY_NOTICE_MS);
+  }
+
+  onBeforeUnmount(() => {
+    if (copyTimer !== undefined) clearTimeout(copyTimer);
+  });
+
+  // reactive 包装解包 ref/computed：模板与调用方直接读布尔（同 useLikes 约定）。
+  return reactive({
+    busy,
+    failed,
+    wechatGuide,
+    copyNotice,
+    canDownload,
+    canDownloadOriginal,
+    shareUrl,
+    download,
+    dismissWechatGuide,
+    copyShareLink,
+  });
+}
+
+export type DownloadController = ReturnType<typeof useDownload>;
diff --git a/apps/viewer/src/i18n/locales/en.ts b/apps/viewer/src/i18n/locales/en.ts
index 5bcee86..8ad7bcc 100644
--- a/apps/viewer/src/i18n/locales/en.ts
+++ b/apps/viewer/src/i18n/locales/en.ts
@@ -19,6 +19,20 @@ export default {
   },
   lang: { switch: 'Switch language' },
   prefs: { reduceMotion: 'Reduce motion' },
+  download: {
+    action: 'Download photo',
+    original: 'Download original',
+    options: 'Download options',
+    busy: 'Preparing…',
+    failed: 'Download failed, please try again',
+    // WeChat in-app browser guidance (no programmatic download): long-press save + copy link.
+    wechatTitle: 'How to save this photo',
+    wechatHint: 'Pages opened inside WeChat cannot download directly. Long-press the photo above to save it, or copy the link and open it in a browser to download.',
+    copyLink: 'Copy link',
+    copied: 'Link copied',
+    copyFailed: 'Copy failed — long-press to select the link below',
+    dismiss: 'Got it',
+  },
   viewer: {
     title: 'Photo viewer',
     close: 'Close',
diff --git a/apps/viewer/src/i18n/locales/zh-CN.ts b/apps/viewer/src/i18n/locales/zh-CN.ts
index 3d9d331..d5dd697 100644
--- a/apps/viewer/src/i18n/locales/zh-CN.ts
+++ b/apps/viewer/src/i18n/locales/zh-CN.ts
@@ -19,6 +19,20 @@ export default {
   },
   lang: { switch: '切换语言' },
   prefs: { reduceMotion: '减少动画' },
+  download: {
+    action: '下载图片',
+    original: '下载原图',
+    options: '下载选项',
+    busy: '准备中…',
+    failed: '下载失败，请重试',
+    // 微信内置浏览器引导（不支持程序化下载）：长按保存 + 复制链接离场。
+    wechatTitle: '保存图片指引',
+    wechatHint: '微信内打开的页面不支持直接下载。请长按上方图片保存到手机，或复制链接后在浏览器中打开再下载。',
+    copyLink: '复制链接',
+    copied: '已复制链接',
+    copyFailed: '复制失败，请长按选择下方链接复制',
+    dismiss: '知道了',
+  },
   viewer: {
     title: '照片查看',
     close: '关闭',
diff --git a/apps/viewer/src/pages/ActivityPage.vue b/apps/viewer/src/pages/ActivityPage.vue
index e2e3cbb..f686a83 100644
--- a/apps/viewer/src/pages/ActivityPage.vue
+++ b/apps/viewer/src/pages/ActivityPage.vue
@@ -12,6 +12,7 @@ import {
   type SortMode,
 } from '../api/client';
 import { errorMessageKey } from '../api/errors';
+import { useDownload } from '../download/useDownload';
 import LikeBubbles from '../components/LikeBubbles.vue';
 import PhotoFeed from '../components/PhotoFeed.vue';
 import PhotoLightbox from '../components/PhotoLightbox.vue';
@@ -222,6 +223,13 @@ const currentPhoto = computed(() => {
   return sequence.photos.find((photo) => photo.photoId === lightboxPhotoId.value) ?? null;
 });
 
+/** 下载控制器（P4a Task 4）：开关来自相册载荷，当前照片随大图键控。 */
+const download = useDownload({
+  token,
+  album: () => album.value,
+  photoId: () => lightboxPhotoId.value,
+});
+
 function showRemovedNotice(): void {
   photoRemovedNotice.value = true;
   if (removedNoticeTimer !== undefined) clearTimeout(removedNoticeTimer);
@@ -444,6 +452,7 @@ onBeforeUnmount(() => {
       v-model:photo-id="lightboxPhotoId"
       :sequence="sequence"
       :likes="likes"
+      :download="download"
       @removed="onPhotoRemoved"
       @close="closeLightbox"
     />
diff --git a/packages/contracts/openapi.yaml b/packages/contracts/openapi.yaml
index 0d4e62f..a8cac72 100644
--- a/packages/contracts/openapi.yaml
+++ b/packages/contracts/openapi.yaml
@@ -3844,6 +3844,8 @@ components:
         - categories
         - banner
         - viewCount
+        - downloadEnabled
+        - originalDownloadEnabled
       properties:
         publicToken:
           type: string
@@ -3889,6 +3891,18 @@ components:
             累计浏览数。cn 部署返回 null（P1a 占位，Task 9 起由统计副本回填）；
             ov 部署返回 album_stats 权威累计值（无行默认 0；计数由 /visits
             端点维护，技术实施方案 §7.1）。
+        downloadEnabled:
+          type: boolean
+          readOnly: true
+          description: |
+            浏览图下载开关（P4a Task 4 / A18）：false 时观众端不出现下载入口，
+            download 端点 browse/original 均 403 DOWNLOAD_DISABLED。
+        originalDownloadEnabled:
+          type: boolean
+          readOnly: true
+          description: |
+            原图下载开关（更高级：需与 downloadEnabled 同时开启才允许
+            variant=original；单独开启不生效——两级级联语义）。
 
     PublicPhoto:
       type: object
diff --git a/packages/contracts/src/constants.ts b/packages/contracts/src/constants.ts
index 6a7eaac..e7615d8 100644
--- a/packages/contracts/src/constants.ts
+++ b/packages/contracts/src/constants.ts
@@ -33,5 +33,9 @@ export const LIKE_BURST_MERGE_MS = 500;
 export const VISIT_REQUEST_TTL_MINUTES = 35;
 /** 浏览上报基础限速：visitorId+IP 固定窗（=去重窗口）内请求数上限，超限 429。 */
 export const VISIT_WINDOW_REQUESTS = 60;
+/** 公开照片下载基础限速：来源 IP 固定窗（DOWNLOAD_WINDOW_SECONDS）内请求数上限。 */
+export const DOWNLOAD_WINDOW_REQUESTS = 120;
+/** 下载限速固定窗秒数（P4a Task 4；无幂等面可借窗，独立短窗）。 */
+export const DOWNLOAD_WINDOW_SECONDS = 60;
 /** ov 互动流水清理调度间隔分钟（like_batches/visit_requests/visit_sessions）。 */
 export const CLEANUP_INTERVAL_MINUTES = 60;
diff --git a/packages/contracts/src/generated/openapi.d.ts b/packages/contracts/src/generated/openapi.d.ts
index 97e7518..08b4bed 100644
--- a/packages/contracts/src/generated/openapi.d.ts
+++ b/packages/contracts/src/generated/openapi.d.ts
@@ -2006,6 +2006,16 @@ export interface components {
              *     端点维护，技术实施方案 §7.1）。
              */
             readonly viewCount: number | null;
+            /**
+             * @description 浏览图下载开关（P4a Task 4 / A18）：false 时观众端不出现下载入口，
+             *     download 端点 browse/original 均 403 DOWNLOAD_DISABLED。
+             */
+            readonly downloadEnabled: boolean;
+            /**
+             * @description 原图下载开关（更高级：需与 downloadEnabled 同时开启才允许
+             *     variant=original；单独开启不生效——两级级联语义）。
+             */
+            readonly originalDownloadEnabled: boolean;
         };
         /** @description 公开照片条目（仅 published 照片，处理完成即有宽高；URL 为短期签名，需在过期前刷新）。 */
         PublicPhoto: {
diff --git a/services/api/src/app-ov.module.ts b/services/api/src/app-ov.module.ts
index c638ad1..0760528 100644
--- a/services/api/src/app-ov.module.ts
+++ b/services/api/src/app-ov.module.ts
@@ -9,7 +9,8 @@
 // 分发见 main-ov 的 ContractWsAdapter），EventEmitter2 全局总线承接「事务提交
 // → 实时广播」（ov-events.ts，单进程——多实例 P5）；Task 8 起 interactions
 // 增浏览上报（visits：窗口去重 + requestId 幂等）并加入 cleanup（互动流水
-// 超窗口行回收调度）。cn 模块保持原样，两入口互不影响。
+// 超窗口行回收调度）；P4a Task 4 起 interactions 增公开照片下载（downloads：
+// 两级开关 + 水印模式键 browse + 原图现签凭证）。cn 模块保持原样，两入口互不影响。
 import {
   type MiddlewareConsumer,
   Module,
diff --git a/services/api/src/interactions/downloads.controller.ts b/services/api/src/interactions/downloads.controller.ts
new file mode 100644
index 0000000..dbab909
--- /dev/null
+++ b/services/api/src/interactions/downloads.controller.ts
@@ -0,0 +1,49 @@
+// services/api/src/interactions/downloads.controller.ts —— 公开照片下载端点
+// （P4a Task 4，ov 域）。路由与契约 downloadPublicPhoto 一致（tags
+// [interactions]，路径前缀不变、部署域名区分）；匿名（无 SessionGuard）。
+// IP 取 req.ip 并归一 IPv6 映射前缀（同 likes.controller，仅作限速键）。
+// variant 查询参数手动解析（同 public-ov parsePublicPhotoSort 口径）：
+// 缺失/非法 → 400 VALIDATION_FAILED（契约 DownloadVariant 枚举外无静默缺省
+// ——下载是显式动作，不允许「缺省猜测版本」）。
+import {
+  Controller,
+  Get,
+  Ip,
+  Param,
+  Query,
+} from '@nestjs/common';
+import { validationFailed } from '../albums/support';
+import {
+  type DownloadVariant,
+  type PublicPhotoDownloadResponse,
+  DownloadsService,
+} from './downloads.service';
+
+/** variant 查询参数解析：仅 browse|original；其余一律 400。 */
+export function parseDownloadVariant(raw: string | undefined): DownloadVariant {
+  if (raw === 'browse' || raw === 'original') {
+    return raw;
+  }
+  throw validationFailed('variant 必须是 browse 或 original');
+}
+
+@Controller('public/albums')
+export class DownloadsController {
+  constructor(private readonly downloads: DownloadsService) {}
+
+  @Get(':token/photos/:photoId/download')
+  download(
+    @Param('token') token: string,
+    @Param('photoId') photoId: string,
+    @Query('variant') variant: string | undefined,
+    @Ip() ip: string | undefined,
+  ): Promise<PublicPhotoDownloadResponse> {
+    const clientIp = ip === undefined ? undefined : ip.replace(/^::ffff:/, '');
+    return this.downloads.download(
+      token,
+      photoId,
+      parseDownloadVariant(variant),
+      clientIp,
+    );
+  }
+}
diff --git a/services/api/src/interactions/downloads.service.ts b/services/api/src/interactions/downloads.service.ts
new file mode 100644
index 0000000..52bc5a6
--- /dev/null
+++ b/services/api/src/interactions/downloads.service.ts
@@ -0,0 +1,157 @@
+// services/api/src/interactions/downloads.service.ts —— 公开照片下载（P4a
+// Task 4，ov 域）。契约 GET /public/albums/{token}/photos/{photoId}/download
+// ?variant=browse|original（downloadPublicPhoto，T12 收口）。全流程：
+//
+//   ① 校验（同 likes/public-ov 读路径口径）：相册 publicToken → albums_ov +
+//      isAlbumVisibleOv（含紧急停用）→ 统一 404 ALBUM_NOT_PUBLIC；照片不存在/
+//      不属于该册/非 published → 404 PHOTO_NOT_PUBLIC；variant 取值非法 →
+//      400 VALIDATION_FAILED（controller 解析）。
+//   ② 基础限速：来源 IP 固定窗（Redis ov INCR + TTL，DOWNLOAD_WINDOW_REQUESTS/
+//      DOWNLOAD_WINDOW_SECONDS）超限 429 RATE_LIMITED（拒绝也消耗预算——固定窗
+//      惯例；无 sessionId/幂等面可借键，纯 IP 键）。NAT 出口共享：上限宽于单
+//      用户直觉点击量（120/分），定位是「误客户端缓冲」，真实防护归 P5 速率层。
+//   ③ 两级开关校验（A18）：browse 需 downloadEnabled；original 需
+//      downloadEnabled **且** originalDownloadEnabled（更高级开关联联）。
+//      对应级别关闭 → 403 DOWNLOAD_DISABLED（browse 级关闭时 original 也 403
+//      ——级联语义，不给「关浏览却开原图」的旁路）。
+//   ④ 键解析：browse = ovBrowseKey（shared 单一实现：水印开启且有效版本 > 0 →
+//      模式键 watermark.jpg，与公开列表/单图同一对象）；original =
+//      photos_ov.originalObjectKey（原图零触碰水印链——A20）。
+//   ⑤ 现签短期 GET 凭证（SIGNED_URL_TTL_SECONDS，SigV4 本地计算）→
+//      {url, expiresAt, variant}。
+//
+// 排除规则（契约 description 落地）：本端点**不**做浏览计数——不写 visit_* /
+// album_stats / outbox_stat_events（e2e 断言行数不变；下载不是浏览）。
+// 日志合规：响应体签名 URL 不入日志（pino 默认只记请求行；本服务亦不显式
+// 打印 url——契约「禁止持久保存/写入分享链接」的服务端面）。
+import { HttpStatus, Inject, Injectable } from '@nestjs/common';
+import {
+  DOWNLOAD_WINDOW_REQUESTS,
+  DOWNLOAD_WINDOW_SECONDS,
+  ErrorCodes,
+} from '@photo/contracts';
+import type { components } from '@photo/contracts';
+import type { StoragePort } from '@photo/storage';
+import { ApiException } from '../api-exception';
+import { albumNotPublic, photoNotPublic } from '../public/support';
+import { OvPrismaService } from '../prisma-ov/ov-prisma.module';
+import { isAlbumVisibleOv } from '../shared/album-visibility';
+import { ovBrowseKey } from '../shared/ov-browse-key';
+import { OvRedisService } from '../redis-ov/ov-redis.module';
+import { signedUrlTtlSeconds } from '../uploads/support';
+import { STORAGE_PORT_OV } from '../public-ov/storage-ov.provider';
+
+export type PublicPhotoDownloadResponse =
+  components['schemas']['PublicPhotoDownload'];
+
+export type DownloadVariant = 'browse' | 'original';
+
+/** 单 IP 固定窗请求数上限（惰性读 env，同 visitWindowRequests 模式）。 */
+export function downloadWindowRequests(): number {
+  const parsed = Number.parseInt(process.env.DOWNLOAD_WINDOW_REQUESTS ?? '', 10);
+  return Number.isFinite(parsed) && parsed >= 0
+    ? parsed
+    : DOWNLOAD_WINDOW_REQUESTS;
+}
+
+function downloadDisabled(): ApiException {
+  return new ApiException(
+    ErrorCodes.DOWNLOAD_DISABLED,
+    '该相册未开放下载',
+    HttpStatus.FORBIDDEN,
+  );
+}
+
+function rateLimited(): ApiException {
+  return new ApiException(
+    ErrorCodes.RATE_LIMITED,
+    '下载过于频繁，请稍后再试',
+    HttpStatus.TOO_MANY_REQUESTS,
+  );
+}
+
+@Injectable()
+export class DownloadsService {
+  constructor(
+    private readonly prisma: OvPrismaService,
+    private readonly redis: OvRedisService,
+    @Inject(STORAGE_PORT_OV) private readonly storage: StoragePort,
+  ) {}
+
+  async download(
+    token: string,
+    photoId: string,
+    variant: DownloadVariant,
+    clientIp: string | undefined,
+  ): Promise<PublicPhotoDownloadResponse> {
+    // ① 相册/照片公开校验（同 likes ① 口径）。
+    const album = await this.prisma.albums_ov.findUnique({
+      where: { publicToken: token },
+    });
+    if (album === null || !isAlbumVisibleOv(album)) {
+      throw albumNotPublic();
+    }
+    const photo = await this.prisma.photos_ov.findUnique({
+      where: { photoId },
+    });
+    if (
+      photo === null ||
+      photo.albumId !== album.albumId ||
+      photo.visibility !== 'published'
+    ) {
+      throw photoNotPublic();
+    }
+
+    // ② 基础限速（通过后才现签；拒绝也消耗预算——固定窗惯例）。
+    await this.assertRateLimit(clientIp);
+
+    // ③ 两级开关（A18）：browse 级关闭 → 两 variant 均 403（级联，无旁路）。
+    if (!album.downloadEnabled) {
+      throw downloadDisabled();
+    }
+    if (variant === 'original' && !album.originalDownloadEnabled) {
+      throw downloadDisabled();
+    }
+
+    // ④ 键解析：browse 复用公开读同一推导（水印模式键一致性）；original 原图键。
+    const objectKey =
+      variant === 'browse'
+        ? ovBrowseKey(
+            {
+              albumId: album.albumId,
+              enabled: album.watermarkEnabled,
+              configVersion: album.watermarkConfigVersion,
+            },
+            photo.photoId,
+            photo.browseObjectKey,
+          )
+        : photo.originalObjectKey;
+
+    // ⑤ 现签短期凭证（ov 只读端口；SigV4 本地计算无网络副作用）。
+    const ttl = signedUrlTtlSeconds();
+    const url = await this.storage.presignGet(objectKey, ttl);
+    return {
+      url,
+      expiresAt: new Date(Date.now() + ttl * 1000).toISOString(),
+      variant,
+    };
+  }
+
+  /** 固定窗限速：Redis ov INCR 计请求数，首次写键挂窗口 TTL（同 visits ③）。 */
+  private async assertRateLimit(clientIp: string | undefined): Promise<void> {
+    if (clientIp === undefined || clientIp === '') {
+      return; // IP 不可得（本地直连测试面）→ 不限速，与 likes IP 键缺席语义一致
+    }
+    const key = `download:window:ip:${clientIp}`;
+    const counted = await this.redis.incr(key);
+    if (counted === 1) {
+      // 新键（本窗首次）：挂 TTL；EXPIRE 失败不影响本轮判定。
+      await this.redis
+        .expire(key, DOWNLOAD_WINDOW_SECONDS)
+        .catch(() => undefined);
+    }
+    if (counted > downloadWindowRequests()) {
+      throw rateLimited();
+    }
+  }
+}
diff --git a/services/api/src/interactions/interactions.module.ts b/services/api/src/interactions/interactions.module.ts
index 140039b..577a9fd 100644
--- a/services/api/src/interactions/interactions.module.ts
+++ b/services/api/src/interactions/interactions.module.ts
@@ -1,20 +1,25 @@
 // services/api/src/interactions/ —— ov 公开互动模块（P2 Task 7 起）。
 // 仅 ov 入口装配（app-ov.module 显式挑选；cn 入口不感知）。Task 7 装配
-// 点赞批次端点；Task 8 装配浏览上报（visits.service/controller）。
+// 点赞批次端点；Task 8 装配浏览上报（visits.service/controller）；P4a Task 4
+// 装配公开照片下载（downloads.service/controller——两级开关 + 现签凭证，
+// STORAGE_PORT_OV 经 StorageOvModule 共享 public-ov 同一实例）。
 // 事件广播经 EventEmitter2（app 根模块 EventEmitterModule.forRoot()
 // 全局注册）发往 realtime 网关（likes.burst / stats.updated），无模块间
 // 直接依赖。
 import { Module } from '@nestjs/common';
 import { OvPrismaModule } from '../prisma-ov/ov-prisma.module';
 import { OvRedisModule } from '../redis-ov/ov-redis.module';
+import { StorageOvModule } from '../public-ov/storage-ov.module';
+import { DownloadsController } from './downloads.controller';
+import { DownloadsService } from './downloads.service';
 import { LikesController } from './likes.controller';
 import { LikesService } from './likes.service';
 import { VisitsController } from './visits.controller';
 import { VisitsService } from './visits.service';
 
 @Module({
-  imports: [OvPrismaModule, OvRedisModule],
-  controllers: [LikesController, VisitsController],
-  providers: [LikesService, VisitsService],
+  imports: [OvPrismaModule, OvRedisModule, StorageOvModule],
+  controllers: [LikesController, VisitsController, DownloadsController],
+  providers: [LikesService, VisitsService, DownloadsService],
 })
 export class InteractionsModule {}
diff --git a/services/api/src/public-ov/public-ov.module.ts b/services/api/src/public-ov/public-ov.module.ts
index 81f4d26..e05afe8 100644
--- a/services/api/src/public-ov/public-ov.module.ts
+++ b/services/api/src/public-ov/public-ov.module.ts
@@ -1,17 +1,18 @@
 // services/api/src/public-ov/public-ov.module.ts —— ov 公开（观众端）读模块
 // （P2 Task 6）。仅 ov 入口装配（app-ov.module 显式挑选；cn 的 PublicModule
 // 互不影响）。装配：PublicOvController/PublicOvService + OvPrismaModule
-// （非 @Global，显式 import）+ STORAGE_PORT_OV（ov 只读存储约定，见
-// storage-ov.provider 头注）。无认证语义（公开端点匿名）。
+// （非 @Global，显式 import）+ StorageOvModule（STORAGE_PORT_OV，ov 只读存储
+// 约定见 storage-ov.provider 头注；P4a Task 4 起独立模块与 interactions 共享
+// 同一实例）。无认证语义（公开端点匿名）。
 import { Module } from '@nestjs/common';
 import { OvPrismaModule } from '../prisma-ov/ov-prisma.module';
 import { PublicOvController } from './public-ov.controller';
 import { PublicOvService } from './public-ov.service';
-import { storageOvProvider } from './storage-ov.provider';
+import { StorageOvModule } from './storage-ov.module';
 
 @Module({
-  imports: [OvPrismaModule],
+  imports: [OvPrismaModule, StorageOvModule],
   controllers: [PublicOvController],
-  providers: [PublicOvService, storageOvProvider],
+  providers: [PublicOvService],
 })
 export class PublicOvModule {}
diff --git a/services/api/src/public-ov/public-ov.service.ts b/services/api/src/public-ov/public-ov.service.ts
index fac0802..210583c 100644
--- a/services/api/src/public-ov/public-ov.service.ts
+++ b/services/api/src/public-ov/public-ov.service.ts
@@ -38,8 +38,8 @@ import {
 import { OvPrismaService } from '../prisma-ov/ov-prisma.module';
 import { signedUrlTtlSeconds } from '../uploads/support';
 import { albumNotPublic, photoNotPublic } from '../public/support';
+import { ovBrowseKey, type WatermarkContextOv } from '../shared/ov-browse-key';
 import { STORAGE_PORT_OV } from './storage-ov.provider';
-import { watermarkVariantObjectKey } from '@photo/storage';
 import type { StoragePort } from '@photo/storage';
 
 export type PublicAlbumResponse = components['schemas']['PublicAlbum'];
@@ -84,12 +84,8 @@ interface HotSnapshot {
   expiresAt: number;
 }
 
-/** 相册级水印服务上下文（albums_ov 副本位 + 归属 albumId，见 ovBrowseKey）。 */
-interface WatermarkContext {
-  albumId: string;
-  enabled: boolean;
-  configVersion: number;
-}
+/** 相册级水印服务上下文（shared/ov-browse-key 单一实现；P4a Task 4 起下载端点共用）。 */
+type WatermarkContext = WatermarkContextOv;
 
 /**
  * 热门快照 offset 游标：base64url JSON 数字（快照内下一页起始下标）。快照在
@@ -183,6 +179,10 @@ export class PublicOvService {
       banner,
       // 真实值（Task 8 的 /visits 端点维护计数；此处只读列，无行 = 0）。
       viewCount: stats?.viewCount ?? 0,
+      // 下载两级开关（P4a Task 4）：viewer 按此渲染下载入口（仅 ov 域消费，
+      // cn 同形返回——契约 PublicAlbum 必返字段）。
+      downloadEnabled: album.downloadEnabled,
+      originalDownloadEnabled: album.originalDownloadEnabled,
     };
   }
 
@@ -287,7 +287,7 @@ export class PublicOvService {
       height: photo.height,
       thumb: await this.presign(photo.thumbObjectKey),
       browse: await this.presign(
-        this.ovBrowseKey(
+        ovBrowseKey(
           this.watermarkContext(album),
           photo.photoId,
           photo.browseObjectKey,
@@ -383,7 +383,7 @@ export class PublicOvService {
         height: photo.height,
         thumb: await this.presign(photo.thumbObjectKey),
         browse: await this.presign(
-          this.ovBrowseKey(watermark, photo.photoId, photo.browseObjectKey),
+          ovBrowseKey(watermark, photo.photoId, photo.browseObjectKey),
         ),
         likeCount: photo.likeTotal,
         publishedAt: (photo.publishedAt ?? new Date(0)).toISOString(),
@@ -406,27 +406,14 @@ export class PublicOvService {
   }
 
   /**
-   * ov 浏览图对象键（P4a Task 3）：水印开启时用**模式键**——
-   * variants/{photoId}/{watermarkConfigVersion}/watermark.jpg（同一对象空间，
-   * cn worker 直写；ov 无变体副本表，键由 cn 行结构确定）。副本位携带的是
-   * **有效版本**（albums.watermarkEffectiveVersion，仅 done 任务推进）：该版本
-   * 键的存在性由重生成任务的收尾不变量保证（每张已发布照片处理成功或回退拷贝
-   * 旧图到目标键——失败照片保留旧图，A20），disable 版任务完成后 enabled=false
-   * 回落 browse 内联键（水印键对象保留，历史可溯）。
+   * ov 浏览图对象键（P4a Task 3，Task 4 起为 shared 单一实现）：水印开启时用
+   * **模式键**——variants/{photoId}/{watermarkConfigVersion}/watermark.jpg（同一
+   * 对象空间，cn worker 直写；ov 无变体副本表，键由 cn 行结构确定）。副本位
+   * 携带的是**有效版本**（albums.watermarkEffectiveVersion，仅 done 任务推进）：
+   * 该版本键的存在性由重生成任务的收尾不变量保证（每张已发布照片处理成功或
+   * 回退拷贝旧图到目标键——失败照片保留旧图，A20），disable 版任务完成后
+   * enabled=false 回落 browse 内联键（水印键对象保留，历史可溯）。
    */
-  private ovBrowseKey(
-    watermark: WatermarkContext,
-    photoId: string,
-    browseObjectKey: string,
-  ): string {
-    return watermark.enabled && watermark.configVersion > 0
-      ? watermarkVariantObjectKey(
-          watermark.albumId,
-          photoId,
-          watermark.configVersion,
-        )
-      : browseObjectKey;
-  }
 
   /** publicToken → albums_ov 副本；不存在 / 不可见（含紧急停用）统一 404。 */
   private async findPublicAlbumOv(token: string) {
diff --git a/services/api/src/public-ov/storage-ov.module.ts b/services/api/src/public-ov/storage-ov.module.ts
new file mode 100644
index 0000000..3b8999c
--- /dev/null
+++ b/services/api/src/public-ov/storage-ov.module.ts
@@ -0,0 +1,13 @@
+// services/api/src/public-ov/storage-ov.module.ts —— STORAGE_PORT_OV 的模块化
+// 装配（P4a Task 4）。与 OvPrismaModule/OvRedisModule 同款约定：非 @Global、
+// provide + export，ov 角色模块（public-ov / interactions）显式 import 共享
+// **同一适配器实例**——此前 provider 直挂 PublicOvModule，下载端点（interactions
+// 域）也需要现签 GET 凭证，收敛到独立模块避免各持一份 S3 客户端。
+import { Module } from '@nestjs/common';
+import { storageOvProvider, STORAGE_PORT_OV } from './storage-ov.provider';
+
+@Module({
+  providers: [storageOvProvider],
+  exports: [STORAGE_PORT_OV],
+})
+export class StorageOvModule {}
diff --git a/services/api/src/public/public.service.ts b/services/api/src/public/public.service.ts
index a41f7db..d49e536 100644
--- a/services/api/src/public/public.service.ts
+++ b/services/api/src/public/public.service.ts
@@ -145,6 +145,10 @@ export class PublicService {
       banner,
       // P1a 占位：浏览计数由后续 /visits 端点提供（技术实施方案 §7.1）。
       viewCount: null,
+      // 下载两级开关（P4a Task 4 契约 PublicAlbum 新增必返字段）：cn 同形返回
+      // （下载端点本身仅 ov 域装配——观众动作端点，见 interactions 模块）。
+      downloadEnabled: album.downloadEnabled,
+      originalDownloadEnabled: album.originalDownloadEnabled,
     };
   }
 
diff --git a/services/api/src/shared/ov-browse-key.ts b/services/api/src/shared/ov-browse-key.ts
new file mode 100644
index 0000000..fe456e0
--- /dev/null
+++ b/services/api/src/shared/ov-browse-key.ts
@@ -0,0 +1,31 @@
+// services/api/src/shared/ov-browse-key.ts —— ov 侧浏览图对象键的单一推导
+// 实现（P4a Task 4 自 public-ov.service 抽出）：公开读（列表/单图 browse 现签）
+// 与下载端点（variant=browse 的下载键）必须落同一对象——水印开启时两者都是
+// 模式键（watermarkVariantObjectKey），关闭时都是 photos_ov 内联 browse 键。
+// 键由 (albumId, photoId, 有效版本) 确定性构成（ov 无变体副本表）；对象存在性
+// 由 cn 侧 done 任务收尾不变量保证（见 packages/contracts/src/watermark.ts 与
+// public-ov.service 头注）。
+import { watermarkVariantObjectKey } from '@photo/storage';
+
+/** 相册级水印服务上下文（albums_ov 副本位 + 归属 albumId）。 */
+export interface WatermarkContextOv {
+  albumId: string;
+  enabled: boolean;
+  configVersion: number;
+}
+
+/**
+ * ov 浏览图对象键：水印开启（enabled && 有效版本 > 0）→ 模式键
+ * variants/{photoId}/{有效版本}/watermark.jpg（cn worker 直写同一对象空间，
+ * 失败照片由任务收尾回退拷贝旧图——键恒有对象）；否则 → photos_ov 内联
+ * browse 键（disable 版任务完成后回落，水印键对象保留历史可溯）。
+ */
+export function ovBrowseKey(
+  watermark: WatermarkContextOv,
+  photoId: string,
+  browseObjectKey: string,
+): string {
+  return watermark.enabled && watermark.configVersion > 0
+    ? watermarkVariantObjectKey(watermark.albumId, photoId, watermark.configVersion)
+    : browseObjectKey;
+}
diff --git a/services/api/test/downloads.e2e-spec.ts b/services/api/test/downloads.e2e-spec.ts
new file mode 100644
index 0000000..1d590a9
--- /dev/null
+++ b/services/api/test/downloads.e2e-spec.ts
@@ -0,0 +1,460 @@
+// services/api/test/downloads.e2e-spec.ts —— P4a Task 4 公开照片下载 e2e
+// （T12 收口）。与 likes-realtime.e2e-spec 同模式：进程内起 cn + ov 两个真实
+// 入口，相册/开关/上传全走 cn API（PATCH albums 开关 → album.updated 同步 →
+// ov 副本生效），OutboxSender（生产实现）指向 ov 入口真实 URL。覆盖（A18）：
+//   ① 开关矩阵：on/on → browse/original 双 200（variant 回显、expiresAt 未来、
+//      URL 可取回真实字节、original 字节 == 上传原图、browse 字节 == browse
+//      变体对象）；on/off → browse 200 / original 403 DOWNLOAD_DISABLED；
+//      off/off 与 off/on（级联——downloadEnabled 关则 original 也 403，无旁路）
+//      → 双 variant 均 403；
+//   ② 水印相册：browse 下载 URL = 模式键 variants/{photoId}/{v}/watermark.jpg
+//      （与公开列表同一 ovBrowseKey 推导），字节 == 水印变体对象；
+//   ③ 可见性：隐藏照片（cn 隐藏 + 同步）→ 404 PHOTO_NOT_PUBLIC；幽灵 token →
+//      404 ALBUM_NOT_PUBLIC；幽灵照片 → 404 PHOTO_NOT_PUBLIC；variant 缺失/
+//      非法 → 400 VALIDATION_FAILED；
+//   ④ 基础限速：DOWNLOAD_WINDOW_REQUESTS 压小（惰性 env）+ 清窗键 → 窗口超额
+//      429 RATE_LIMITED；
+//   ⑤ 排除规则：全程下载调用后 visit_requests / album_stats 零行（下载不计数）。
+import { NestFactory } from '@nestjs/core';
+import { type NestExpressApplication } from '@nestjs/platform-express';
+import { Readable } from 'node:stream';
+import request from 'supertest';
+import sharp from 'sharp';
+import Redis from 'ioredis';
+import { PrismaClient } from '@photo/db-cn';
+import { PrismaClient as OvPrismaClient } from '@photo/db-ov';
+import { ErrorCodes } from '@photo/contracts';
+import {
+  createStorageFromEnv,
+  type MinioStorageAdapter,
+} from '@photo/storage';
+import { AppCnModule } from '../src/app-cn.module';
+import { AppOvModule } from '../src/app-ov.module';
+import { seedAdmin } from '../src/scripts/seed-admin';
+import { ContractWsAdapter } from '../src/realtime/contract-ws.adapter';
+import { configureApp } from '../src/shared/configure-app';
+import { PublishService } from '../../worker/src/publish.service';
+import { WatermarkRegenService } from '../../worker/src/watermark.service';
+import { resetDb } from './reset';
+import { resetOvDb } from './reset-ov';
+import { OutboxSender } from '../../sync/src/sender';
+
+jest.setTimeout(180_000);
+
+const ADMIN_ACCOUNT = process.env.ADMIN_ACCOUNT ?? 'admin';
+const ADMIN_PASSWORD = process.env.ADMIN_PASSWORD ?? 'admin-secret-change-me';
+const SYNC_KEY = process.env.SYNC_INTERNAL_KEY ?? 'dev-sync-secret-change-me';
+
+/** 测试内生成真实 JPEG（无二进制入库）。 */
+async function makeJpeg(
+  width: number,
+  height: number,
+  rgb: { r: number; g: number; b: number },
+): Promise<Buffer> {
+  return sharp({ create: { width, height, channels: 3, background: rgb } })
+    .jpeg({ quality: 85 })
+    .toBuffer();
+}
+
+interface SeededAlbum {
+  albumId: string;
+  token: string;
+}
+
+describe('downloads (e2e)', () => {
+  let cnApp: NestExpressApplication;
+  let ovApp: NestExpressApplication;
+  let cnBase: string;
+  let ovBase: string;
+  let prismaCn: PrismaClient;
+  let prismaOv: OvPrismaClient;
+  let storage: MinioStorageAdapter;
+  let sender: OutboxSender;
+  let adminToken: string;
+
+  /** 四开关组合相册 + 水印相册（各含已发布照片；on/on 另含隐藏目标）。 */
+  let onOn: SeededAlbum;
+  let onOff: SeededAlbum;
+  let offOff: SeededAlbum;
+  let offOn: SeededAlbum;
+  let watermarked: SeededAlbum;
+  let onOnPhoto: string;
+  let onOnHidden: string;
+  let onOffPhoto: string;
+  let offOffPhoto: string;
+  let offOnPhoto: string;
+  let wmPhoto: string;
+  /** 上传原图字节（original 下载对照基准）。 */
+  let onOnOriginal: Buffer;
+
+  /** 排空发送器：循环 runOnce 直至无未投递事件。 */
+  async function drainSender(): Promise<void> {
+    for (let round = 0; round < 20; round++) {
+      const stats = await sender.runOnce();
+      if (stats.picked === 0) {
+        return;
+      }
+    }
+    throw new Error('drainSender 超时：20 轮后 outbox 仍有未投递事件');
+  }
+
+  /** Redis ov 清限速窗键（跨 spec 残留 / 测试间重置计数）。 */
+  async function clearDownloadWindowKeys(): Promise<void> {
+    const redis = new Redis(
+      process.env.REDIS_URL_OV ?? 'redis://127.0.0.1:6381',
+    );
+    try {
+      let cursor = '0';
+      do {
+        const [next, keys] = await redis.scan(
+          cursor,
+          'MATCH',
+          'download:window:*',
+          'COUNT',
+          1000,
+        );
+        cursor = next;
+        if (keys.length > 0) {
+          await redis.del(...keys);
+        }
+      } while (cursor !== '0');
+    } finally {
+      await redis.quit();
+    }
+  }
+
+  /** cn 建相册（缺省开关 true/false）→ 可选 PATCH 开关 → 返回 {albumId, token}。 */
+  async function createAlbum(
+    title: string,
+    switches?: { downloadEnabled?: boolean; originalDownloadEnabled?: boolean },
+  ): Promise<SeededAlbum> {
+    const album = await request(cnBase)
+      .post('/v1/albums')
+      .set('Authorization', `Bearer ${adminToken}`)
+      .send({
+        translations: [{ locale: 'zh-CN', title }],
+        defaultLocale: 'zh-CN',
+        startAt: '2026-12-01T10:00:00.000Z',
+      });
+    expect(album.status).toBe(201);
+    const seeded = {
+      albumId: album.body.id as string,
+      token: album.body.publicToken as string,
+    };
+    if (switches !== undefined) {
+      // 开关经 cn PATCH（album.updated 事件 → ov 副本；比直改副本行诚实）。
+      const patch = await request(cnBase)
+        .patch(`/v1/albums/${seeded.albumId}`)
+        .set('Authorization', `Bearer ${adminToken}`)
+        .send(switches);
+      expect(patch.status).toBe(200);
+    }
+    return seeded;
+  }
+
+  /** 上传全链路：init → presign PUT 实写 MinIO → complete → 直驱发布事务。 */
+  async function uploadAndPublish(
+    albumId: string,
+    clientTaskId: string,
+    jpeg: Buffer,
+  ): Promise<string> {
+    const init = await request(cnBase)
+      .post('/v1/uploads')
+      .set('Authorization', `Bearer ${adminToken}`)
+      .send({
+        albumId,
+        clientTaskId,
+        filename: 'download.jpg',
+        bytes: jpeg.length,
+        contentType: 'image/jpeg',
+      });
+    expect(init.status).toBe(201);
+    const { uploadId, photoId, credential } = init.body as {
+      uploadId: string;
+      photoId: string;
+      credential: { url: string; method: string; headers: Record<string, string> };
+    };
+    const put = await fetch(credential.url, {
+      method: credential.method,
+      headers: credential.headers,
+      body: new Uint8Array(jpeg),
+    });
+    expect(put.ok).toBe(true);
+    const done = await request(cnBase)
+      .post(`/v1/uploads/${uploadId}/complete`)
+      .set('Authorization', `Bearer ${adminToken}`);
+    expect(done.status).toBe(200);
+    expect(await new PublishService(prismaCn, storage).processPhoto(photoId)).toBe(
+      'published',
+    );
+    return photoId;
+  }
+
+  /** 下载 GET（返回 supertest Response）。 */
+  function download(
+    album: SeededAlbum,
+    photoId: string,
+    variant: string,
+  ): Promise<request.Response> {
+    return request(ovBase).get(
+      `/v1/public/albums/${album.token}/photos/${photoId}/download?variant=${variant}`,
+    );
+  }
+
+  /** cn browse 变体对象字节（browse 下载对照基准）。 */
+  async function browseObjectOf(photoId: string): Promise<Buffer> {
+    const row = await prismaCn.photo_variants.findUniqueOrThrow({
+      where: { photoId_variant: { photoId, variant: 'browse' } },
+    });
+    return readObject(row.objectKey);
+  }
+
+  /** 流式读取对象为单缓冲（与 worker readOriginal 同约定）。 */
+  async function readObject(key: string): Promise<Buffer> {
+    const stream = (await storage.getStream(key)) as Readable;
+    const chunks: Buffer[] = [];
+    for await (const chunk of stream) {
+      chunks.push(Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk as Uint8Array));
+    }
+    return Buffer.concat(chunks);
+  }
+
+  beforeAll(async () => {
+    expect(SYNC_KEY).not.toBe('');
+    await resetDb();
+    await resetOvDb();
+    await clearDownloadWindowKeys();
+
+    cnApp = await NestFactory.create<NestExpressApplication>(AppCnModule, {
+      rawBody: true,
+    });
+    configureApp(cnApp);
+    await cnApp.listen(0);
+    cnBase = await cnApp.getUrl();
+
+    ovApp = await NestFactory.create<NestExpressApplication>(AppOvModule, {
+      rawBody: true,
+    });
+    // main-ov 同形：契约帧适配器（WS 与 HTTP 同端口；本 spec 不用 WS，仅为
+    // 与生产入口同形的装配）。
+    ovApp.useWebSocketAdapter(new ContractWsAdapter(ovApp));
+    configureApp(ovApp);
+    await ovApp.listen(0);
+    ovBase = await ovApp.getUrl();
+
+    prismaCn = new PrismaClient();
+    prismaOv = new OvPrismaClient();
+    storage = createStorageFromEnv();
+    sender = new OutboxSender({
+      prisma: prismaCn,
+      ovApiUrl: ovBase,
+      internalKey: SYNC_KEY,
+    });
+
+    await seedAdmin(prismaCn, ADMIN_ACCOUNT, ADMIN_PASSWORD);
+    const login = await request(cnBase).post('/v1/auth/password').send({
+      account: ADMIN_ACCOUNT,
+      password: ADMIN_PASSWORD,
+    });
+    expect(login.status).toBe(200);
+    adminToken = login.body.sessionToken as string;
+
+    // —— 开关矩阵四相册（缺省 true/false；PATCH 显式设定目标组合）——
+    onOn = await createAlbum('下载开原图开', {
+      originalDownloadEnabled: true,
+    });
+    onOff = await createAlbum('下载开原图关');
+    offOff = await createAlbum('下载关原图关', {
+      downloadEnabled: false,
+      originalDownloadEnabled: false,
+    });
+    offOn = await createAlbum('下载关原图开', {
+      downloadEnabled: false,
+      originalDownloadEnabled: true,
+    });
+
+    onOnOriginal = await makeJpeg(960, 640, { r: 190, g: 90, b: 40 });
+    onOnPhoto = await uploadAndPublish(onOn.albumId, 'dl-onon', onOnOriginal);
+    onOnHidden = await uploadAndPublish(
+      onOn.albumId,
+      'dl-onon-hidden',
+      await makeJpeg(800, 500, { r: 90, g: 190, b: 120 }),
+    );
+    onOffPhoto = await uploadAndPublish(
+      onOff.albumId,
+      'dl-onoff',
+      await makeJpeg(900, 600, { r: 60, g: 120, b: 190 }),
+    );
+    offOffPhoto = await uploadAndPublish(
+      offOff.albumId,
+      'dl-offoff',
+      await makeJpeg(700, 450, { r: 120, g: 60, b: 190 }),
+    );
+    offOnPhoto = await uploadAndPublish(
+      offOn.albumId,
+      'dl-offon',
+      await makeJpeg(850, 550, { r: 60, g: 190, b: 190 }),
+    );
+
+    // 隐藏目标：cn 隐藏 → photo.hidden（高优先级）→ ov 撤下。
+    const hide = await request(cnBase)
+      .patch(`/v1/photos/${onOnHidden}`)
+      .set('Authorization', `Bearer ${adminToken}`)
+      .send({ visibility: 'hidden' });
+    expect(hide.status).toBe(200);
+
+    // —— 水印相册：POST watermark-jobs（enabled）→ 直驱 done → ov 有效位 ——
+    watermarked = await createAlbum('下载水印相册');
+    wmPhoto = await uploadAndPublish(
+      watermarked.albumId,
+      'dl-wm',
+      await makeJpeg(900, 600, { r: 150, g: 150, b: 60 }),
+    );
+    const wmJob = await request(cnBase)
+      .post(`/v1/albums/${watermarked.albumId}/watermark-jobs`)
+      .set('Authorization', `Bearer ${adminToken}`)
+      .send({ enabled: true, text: { content: '© Download e2e' } });
+    expect(wmJob.status).toBe(202);
+    expect(
+      await new WatermarkRegenService(prismaCn, storage).processJob(
+        wmJob.body.jobId as string,
+      ),
+    ).toBe('done');
+
+    await drainSender();
+  });
+
+  afterAll(async () => {
+    delete process.env.DOWNLOAD_WINDOW_REQUESTS;
+    await ovApp.close();
+    await cnApp.close();
+    await prismaCn.$disconnect();
+    await prismaOv.$disconnect();
+  });
+
+  it('① on/on：双 variant 200、variant/expiresAt 正确、URL 真实字节（original == 上传原图）', async () => {
+    const browse = await download(onOn, onOnPhoto, 'browse');
+    expect(browse.status).toBe(200);
+    expect(browse.body.variant).toBe('browse');
+    expect(Date.parse(browse.body.expiresAt)).toBeGreaterThan(Date.now());
+    // browse 键 = 内联 browse 键（无水印相册）且字节 == browse 变体对象。
+    expect(browse.body.url).toContain(
+      `/variants/${onOnPhoto}/1/browse.jpg`,
+    );
+    const browseBytes = Buffer.from(
+      await (await fetch(browse.body.url)).arrayBuffer(),
+    );
+    expect(browseBytes.length).toBeGreaterThan(0);
+    expect(browseBytes.equals(await browseObjectOf(onOnPhoto))).toBe(true);
+
+    const original = await download(onOn, onOnPhoto, 'original');
+    expect(original.status).toBe(200);
+    expect(original.body.variant).toBe('original');
+    expect(Date.parse(original.body.expiresAt)).toBeGreaterThan(Date.now());
+    expect(original.body.url).toContain(`/originals/${onOnPhoto}.jpg`);
+    // A20 语义的下载面：原图零触碰——字节与上传时完全一致。
+    const originalBytes = Buffer.from(
+      await (await fetch(original.body.url)).arrayBuffer(),
+    );
+    expect(originalBytes.equals(onOnOriginal)).toBe(true);
+  });
+
+  it('② on/off：browse 200；original 403 DOWNLOAD_DISABLED', async () => {
+    const browse = await download(onOff, onOffPhoto, 'browse');
+    expect(browse.status).toBe(200);
+    expect(browse.body.variant).toBe('browse');
+    expect((await fetch(browse.body.url)).ok).toBe(true);
+
+    const original = await download(onOff, onOffPhoto, 'original');
+    expect(original.status).toBe(403);
+    expect(original.body.code).toBe(ErrorCodes.DOWNLOAD_DISABLED);
+  });
+
+  it('③ off/off 与 off/on（级联）：双 variant 均 403 DOWNLOAD_DISABLED', async () => {
+    for (const album of [offOff, offOn]) {
+      const photoId = album === offOff ? offOffPhoto : offOnPhoto;
+      const browse = await download(album, photoId, 'browse');
+      expect(browse.status).toBe(403);
+      expect(browse.body.code).toBe(ErrorCodes.DOWNLOAD_DISABLED);
+      // 级联：downloadEnabled 关 → original 无旁路（即使 originalDownloadEnabled 开）。
+      const original = await download(album, photoId, 'original');
+      expect(original.status).toBe(403);
+      expect(original.body.code).toBe(ErrorCodes.DOWNLOAD_DISABLED);
+    }
+    // ov 副本开关确已同步到位（矩阵前提非巧合）。
+    const ovAlbum = await prismaOv.albums_ov.findUniqueOrThrow({
+      where: { albumId: offOn.albumId },
+    });
+    expect(ovAlbum.downloadEnabled).toBe(false);
+    expect(ovAlbum.originalDownloadEnabled).toBe(true);
+  });
+
+  it('④ 水印相册：browse 下载 = 模式键 watermark.jpg，字节 == 水印变体对象', async () => {
+    const res = await download(watermarked, wmPhoto, 'browse');
+    expect(res.status).toBe(200);
+    expect(res.body.url).toContain(
+      `/variants/${wmPhoto}/1/watermark.jpg`,
+    );
+    const bytes = Buffer.from(
+      await (await fetch(res.body.url)).arrayBuffer(),
+    );
+    const wmRow = await prismaCn.photo_variants.findUniqueOrThrow({
+      where: { photoId_variant: { photoId: wmPhoto, variant: 'watermark' } },
+    });
+    expect(bytes.equals(await readObject(wmRow.objectKey))).toBe(true);
+    // 与公开列表 browse 同键（同一 ovBrowseKey 推导的间接证明）。
+    const list = await request(ovBase)
+      .get(`/v1/public/albums/${watermarked.token}/photos?limit=10`);
+    expect(list.body.items[0].browse.url).toContain(
+      `/variants/${wmPhoto}/1/watermark.jpg`,
+    );
+  });
+
+  it('⑤ 可见性与参数校验：隐藏 404 / 幽灵 token 404 / 幽灵照片 404 / variant 非法 400', async () => {
+    const hidden = await download(onOn, onOnHidden, 'browse');
+    expect(hidden.status).toBe(404);
+    expect(hidden.body.code).toBe(ErrorCodes.PHOTO_NOT_PUBLIC);
+
+    const ghostAlbum = await request(ovBase).get(
+      `/v1/public/albums/${'g' + '0'.repeat(23)}/photos/${onOnPhoto}/download?variant=browse`,
+    );
+    expect(ghostAlbum.status).toBe(404);
+    expect(ghostAlbum.body.code).toBe(ErrorCodes.ALBUM_NOT_PUBLIC);
+
+    const ghostPhoto = await download(onOn, 'p-ghost', 'browse');
+    expect(ghostPhoto.status).toBe(404);
+    expect(ghostPhoto.body.code).toBe(ErrorCodes.PHOTO_NOT_PUBLIC);
+
+    for (const query of ['', '?variant=', '?variant=thumb', '?variant=1']) {
+      const res = await request(ovBase).get(
+        `/v1/public/albums/${onOn.token}/photos/${onOnPhoto}/download${query}`,
+      );
+      expect(res.status).toBe(400);
+      expect(res.body.code).toBe(ErrorCodes.VALIDATION_FAILED);
+    }
+  });
+
+  it('⑥ 基础限速：同 IP 窗口超额 → 429 RATE_LIMITED', async () => {
+    // 清窗键重置计数（①-⑤ 已消耗本机 IP 预算）+ 压小上限。
+    await clearDownloadWindowKeys();
+    process.env.DOWNLOAD_WINDOW_REQUESTS = '3';
+    try {
+      for (let i = 0; i < 3; i++) {
+        const ok = await download(onOn, onOnPhoto, 'browse');
+        expect(ok.status).toBe(200);
+      }
+      const limited = await download(onOn, onOnPhoto, 'browse');
+      expect(limited.status).toBe(429);
+      expect(limited.body.code).toBe(ErrorCodes.RATE_LIMITED);
+      // 拒绝不改写已签发凭证语义：前序 URL 仍可取回（限速只挡新签发）。
+    } finally {
+      delete process.env.DOWNLOAD_WINDOW_REQUESTS;
+      await clearDownloadWindowKeys();
+    }
+  });
+
+  it('⑦ 排除规则：下载不计数——visit_requests / album_stats 全程零行', async () => {
+    expect(await prismaOv.visit_requests.count()).toBe(0);
+    expect(await prismaOv.album_stats.count()).toBe(0);
+  });
+});
diff --git a/services/api/test/public.e2e-spec.ts b/services/api/test/public.e2e-spec.ts
index 0deab27..8c5c70b 100644
--- a/services/api/test/public.e2e-spec.ts
+++ b/services/api/test/public.e2e-spec.ts
@@ -455,12 +455,14 @@ describe('public (e2e)', () => {
       expect(res.body.endAt).toBe('2026-10-01T12:00:00.000Z');
       expect(Number.isNaN(Date.parse(res.body.startAt))).toBe(false);
       expect(Number.isNaN(Date.parse(res.body.endAt))).toBe(false);
-      // 契约 PublicAlbum 必返字段全集（形状精确断言）。
+      // 契约 PublicAlbum 必返字段全集（形状精确断言；P4a Task 4 增下载开关）。
       expect(Object.keys(res.body).sort()).toEqual([
         'banner',
         'categories',
         'defaultLocale',
+        'downloadEnabled',
         'endAt',
+        'originalDownloadEnabled',
         'publicToken',
         'resolvedLocale',
         'startAt',