diff --git a/docs/APP_REFACTORING_DESIGN.md b/docs/APP_REFACTORING_DESIGN.md
index 5b4dc92..00c4cc3 100644
--- a/docs/APP_REFACTORING_DESIGN.md
+++ b/docs/APP_REFACTORING_DESIGN.md
@@ -100,7 +100,7 @@ We will gradually eliminate React state variables in `src/app.tsx` and derive th
 *   [x] **Step 3.4:** Extract `config` state. Parse display settings on render using `argsToConfig` helper. Remove state. In `ViewPage`, run `updateChartWithConfig(config, data)` synchronously during the render pass (e.g. in the `useMemo` that derives query parameters/config) to update the in-memory chart data before it is rendered by the `<Chart>` child component. Run E2E and visual tests to verify no rendering regressions occurred.
 
 #### Phase 4: Structural Page Nesting
-*   [ ] **Step 4.1:** Create `src/hooks/use_google_auth.ts` to manage Google Drive authentication token updates. Remove `hasGoogleToken` state from `src/app.tsx`.
+*   [x] **Step 4.1:** Create `src/hooks/use_google_auth.ts` to manage Google Drive authentication token updates. Remove `hasGoogleToken` state from `src/app.tsx`.
 *   [ ] **Step 4.2:** Create `src/pages/view_page.tsx` containing layout rendering (`renderMainArea`), asynchronous data loaders, view state management, `revokeObjectUrls` cleanups, and `<GoogleAuthModal>` error fallback state and rendering.
 *   [ ] **Step 4.3:** Create `src/pages/intro_page.tsx` rendering the landing screen and its own local header.
 *   [ ] **Step 4.4:** Refactor `src/app.tsx` to serve as a pure routing switch routing to `<IntroPage />` or `<ViewPage />`. In `App`, handle Google Drive redirection synchronously during the render pass by returning a `<Navigate replace />` element (preventing render flashing), and ensure the redirection parser checks both the router `location.search` and the external `window.location.search` to handle HashRouter query structure robustly. Once external parameters (like `state` or `authcode`) are detected, they must be stripped from `window.location.search` using `window.history.replaceState` to prevent redirection loops on page refresh. Remove all layout state logic from the root file, including moving the `useWebMcpBridge` hook invocation into `<ViewPage />`. Run final verification scripts (`npm run check:all`).
diff --git a/src/app.tsx b/src/app.tsx
index b3f167e..fb60ec0 100644
--- a/src/app.tsx
+++ b/src/app.tsx
@@ -43,6 +43,7 @@ import {
   WikiTreeSourceSpec,
 } from './datasource/wikitree';
 import {DonatsoChart} from './donatso-chart';
+import {useGoogleAuth} from './hooks/use_google_auth';
 import {useWebMcpBridge} from './hooks/use_webmcp_bridge';
 import {Intro} from './intro';
 import {GoogleAuthModal} from './menu/google_auth_modal';
@@ -90,10 +91,8 @@ export function App() {
   const [showAuthModal, setShowAuthModal] = useState(false);
   /** Stores the file ID that failed to load from Google Drive due to authorization errors. */
   const [failedFileId, setFailedFileId] = useState<string>();
-  /** Tracks whether the user has a valid cached Google Drive OAuth access token. */
-  const [hasGoogleToken, setHasGoogleToken] = useState(
-    () => !!googleDriveService.getAccessToken(),
-  );
+  /** Tracks whether the user has a valid cached Google Drive OAuth access token and provides a state setter. */
+  const {hasGoogleToken, setHasGoogleToken} = useGoogleAuth();
 
   const intl = useIntl();
   const navigate = useNavigate();
diff --git a/src/hooks/use_google_auth.ts b/src/hooks/use_google_auth.ts
new file mode 100644
index 0000000..19674de
--- /dev/null
+++ b/src/hooks/use_google_auth.ts
@@ -0,0 +1,19 @@
+import {useState} from 'react';
+import {googleDriveService} from '../datasource/google_drive_service';
+
+/**
+ * Custom React hook to manage and track the state of Google Drive OAuth2 credentials.
+ */
+export function useGoogleAuth() {
+  /** Tracks whether the user has a valid cached Google Drive OAuth access token. */
+  const [hasGoogleToken, setHasGoogleToken] = useState(
+    () => !!googleDriveService.getAccessToken(),
+  );
+
+  return {
+    /** Tracks whether the user has a valid cached Google Drive OAuth access token. */
+    hasGoogleToken,
+    /** Function to update the token presence state in React. */
+    setHasGoogleToken,
+  };
+}