diff --git a/apps/pwa/src/routes/SetupPage.test.tsx b/apps/pwa/src/routes/SetupPage.test.tsx
index 80caf21..9634506 100644
--- a/apps/pwa/src/routes/SetupPage.test.tsx
+++ b/apps/pwa/src/routes/SetupPage.test.tsx
@@ -372,4 +372,79 @@ describe('SetupPage — Step 2 Instance copy + DB-name field (gaps 1, 3)', () =>
   });
 });
 
-// (gap-4 Back-navigation tests added in Task 2)
+// ── Tests: Back navigation preserves Instance fields (gap 4) ──────────────────
+
+describe('SetupPage — Back navigation preserves Instance fields (gap 4)', () => {
+  let queryClient: QueryClient;
+
+  beforeEach(async () => {
+    queryClient = makeQueryClient();
+    vi.resetAllMocks();
+    const { fetchSetupStatus } = await import('../api/client.js');
+    (fetchSetupStatus as ReturnType<typeof vi.fn>).mockResolvedValue({
+      setupComplete: false,
+      dbName: 'familysync',
+    });
+  });
+
+  /**
+   * Helper: fill the four Instance fields, run validation to GREEN, advance to
+   * the Calendar step (step 3).
+   */
+  async function fillStep2AndAdvance() {
+    const { postSetupConfig, validateSetupDb, validateSetupOidc, validateSetupVapid } =
+      await import('../api/client.js');
+    (postSetupConfig as ReturnType<typeof vi.fn>).mockResolvedValue(undefined);
+    (validateSetupDb as ReturnType<typeof vi.fn>).mockResolvedValue(undefined);
+    (validateSetupOidc as ReturnType<typeof vi.fn>).mockResolvedValue(undefined);
+    (validateSetupVapid as ReturnType<typeof vi.fn>).mockResolvedValue(undefined);
+
+    renderSetupPage(queryClient);
+    fireEvent.click(await screen.findByRole('button', { name: 'Continue' }));
+    await screen.findByText('Instance Configuration');
+
+    fireEvent.change(screen.getByLabelText('App URL'), { target: { value: 'https://app.example.com' } });
+    fireEvent.change(screen.getByLabelText('OIDC issuer URL'), { target: { value: 'https://auth.example.com' } });
+    fireEvent.change(screen.getByLabelText('OIDC client ID'), { target: { value: 'familysync' } });
+    fireEvent.change(screen.getByLabelText('VAPID public key'), { target: { value: 'BHtest123' } });
+    fireEvent.click(screen.getByRole('button', { name: 'Save & Validate' }));
+
+    // After all validations pass, the Continue button appears
+    fireEvent.click(await screen.findByRole('button', { name: 'Continue' }));
+    await screen.findByText('Fastmail Credential');
+  }
+
+  it('restores all four Instance field values after navigating Back from Calendar step', async () => {
+    await fillStep2AndAdvance();
+
+    // Now on step 3 (Calendar). Navigate Back.
+    fireEvent.click(screen.getByRole('button', { name: 'Back' }));
+    await screen.findByText('Instance Configuration');
+
+    expect(screen.getByLabelText('App URL')).toHaveValue('https://app.example.com');
+    expect(screen.getByLabelText('OIDC issuer URL')).toHaveValue('https://auth.example.com');
+    expect(screen.getByLabelText('OIDC client ID')).toHaveValue('familysync');
+    expect(screen.getByLabelText('VAPID public key')).toHaveValue('BHtest123');
+  });
+
+  it('does NOT persist the Fastmail app password across Back/forward navigation (T-12-15)', async () => {
+    await fillStep2AndAdvance();
+
+    // On step 3: type a password into the app password field.
+    const pwField = screen.getByLabelText('App password');
+    fireEvent.change(pwField, { target: { value: 'super-secret-pw' } });
+    expect(pwField).toHaveValue('super-secret-pw');
+
+    // Back to step 2 — field values are preserved, but validation state is not
+    // lifted, so re-run Save & Validate to surface Continue, then advance to step 3.
+    fireEvent.click(screen.getByRole('button', { name: 'Back' }));
+    await screen.findByText('Instance Configuration');
+    // Fields are still populated (gap 4), so just re-validate.
+    fireEvent.click(screen.getByRole('button', { name: 'Save & Validate' }));
+    fireEvent.click(await screen.findByRole('button', { name: 'Continue' }));
+    await screen.findByText('Fastmail Credential');
+
+    // Step 3 re-mounted with fresh local state — the app password is NOT persisted.
+    expect(screen.getByLabelText('App password')).toHaveValue('');
+  });
+});
diff --git a/apps/pwa/src/routes/SetupPage.tsx b/apps/pwa/src/routes/SetupPage.tsx
index 23da0fa..b5652f3 100644
--- a/apps/pwa/src/routes/SetupPage.tsx
+++ b/apps/pwa/src/routes/SetupPage.tsx
@@ -430,17 +430,28 @@ function Step1Welcome({ onContinue, stepHeadingRef }: Step1Props) {
 
 // ── Step 2: Instance Configuration ───────────────────────────────────────────
 
+interface InstanceFields {
+  appUrl: string;
+  oidcIssuer: string;
+  oidcClientId: string;
+  vapidPublicKey: string;
+}
+
 interface Step2Props {
   onBack: () => void;
   onSuccess: () => void;
   stepHeadingRef: React.RefObject<HTMLHeadingElement | null>;
+  /** Lifted to SetupPage so values survive step unmount (gap 4: Back preserves entries). */
+  fields: InstanceFields;
+  setFields: React.Dispatch<React.SetStateAction<InstanceFields>>;
 }
 
-function Step2Config({ onBack, onSuccess, stepHeadingRef }: Step2Props) {
-  const [appUrl, setAppUrl] = useState('');
-  const [oidcIssuer, setOidcIssuer] = useState('');
-  const [oidcClientId, setOidcClientId] = useState('');
-  const [vapidPublicKey, setVapidPublicKey] = useState('');
+function Step2Config({ onBack, onSuccess, stepHeadingRef, fields, setFields }: Step2Props) {
+  const { appUrl, oidcIssuer, oidcClientId, vapidPublicKey } = fields;
+  const setAppUrl = (v: string) => setFields((f) => ({ ...f, appUrl: v }));
+  const setOidcIssuer = (v: string) => setFields((f) => ({ ...f, oidcIssuer: v }));
+  const setOidcClientId = (v: string) => setFields((f) => ({ ...f, oidcClientId: v }));
+  const setVapidPublicKey = (v: string) => setFields((f) => ({ ...f, vapidPublicKey: v }));
   const [fieldError, setFieldError] = useState<string | null>(null);
 
   // Gap 3 (frontend): fetch the env-derived, non-secret DB name so the
@@ -1072,6 +1083,16 @@ export function SetupPage({ alreadyLocked = false }: SetupPageProps) {
   const [terminal, setTerminal] = useState<TerminalState>(alreadyLocked ? 'locked' : null);
   const stepHeadingRef = useRef<HTMLHeadingElement>(null);
 
+  // Gap 4: Instance-step field values are lifted here so they survive Step2 unmount.
+  // The Fastmail app password (Step 3) is deliberately NOT lifted — it stays in
+  // Step3Credential local state and is cleared on unmount (T-12-15 preserved).
+  const [instanceFields, setInstanceFields] = useState<InstanceFields>({
+    appUrl: '',
+    oidcIssuer: '',
+    oidcClientId: '',
+    vapidPublicKey: '',
+  });
+
   // Focus the step heading on step change for a11y (D-04 focus management)
   useEffect(() => {
     if (stepHeadingRef.current) {
@@ -1166,6 +1187,8 @@ export function SetupPage({ alreadyLocked = false }: SetupPageProps) {
             onBack={() => setStep(1)}
             onSuccess={() => setStep(3)}
             stepHeadingRef={stepHeadingRef}
+            fields={instanceFields}
+            setFields={setInstanceFields}
           />
         )}
         {step === 3 && (