diff --git a/CHANGELOG.md b/CHANGELOG.md
index f0a70869f..2c110b24b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,6 +55,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   so no canonical names a redirect. `og:url` is built by the same function, so
   the two cannot disagree. Nothing is emitted without `site_url`
   ([#207](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/207)) ([#227](https://github.com/QuantEcon/quantecon-theme.mystmd/pull/227)).
+- The "On this page" outline lists h4 subsections, which the Sphinx panel lists
+  and this one left out: a reader inside an h4 saw its h3 marked with nothing
+  below it. The panel is now a tree of any depth, and the Sphinx expansion rule
+  applies at every level of it — the current entry's own sub-list and those of
+  all its ancestors are open, while only the current entry itself is marked.
+  h4 entries are indented one step further than h3s. h5 and deeper are still
+  not listed
+  ([#208](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/208)) ([#228](https://github.com/QuantEcon/quantecon-theme.mystmd/pull/228)).
 
 ### Changed
 - **Breaking: the Launch control is now opt-in and explicitly configured.** It
diff --git a/app/components/Outline.tsx b/app/components/Outline.tsx
index 0f3cce9fd..adcdb4070 100644
--- a/app/components/Outline.tsx
+++ b/app/components/Outline.tsx
@@ -52,10 +52,12 @@ export function BackToTop() {
  *    `sticky`: the wrapper is `self-start`, so a sticky child would have zero
  *    travel, and the grid declares no rows for it to span. `left`/`right` stay
  *    `auto`, so the panel keeps its static position in the margin track.
- *  - h3 entries nest under their h2 and collapse to the active branch: at the
- *    top of the page only the sections show; scrolling into a section expands
- *    its subsections, the current one (section or subsection) is marked, and
- *    the parent of a current subsection is expanded but not marked. Past
+ *  - h2 to h4 nest into a tree of any depth, which collapses to the active
+ *    branch: at the top of the page only the sections show, and a branch opens
+ *    when the current entry is inside it. The rule runs at every level -- the
+ *    current entry's own sub-list and those of all its ancestors are open --
+ *    so a current h3 shows its h4s. Only the current entry is marked, at
+ *    whatever depth it sits; its ancestors are open but unmarked. Past
  *    `max-height` the panel scrolls internally.
  *  - Enumerators come from the heading itself (`span.select-none`, "3.1"),
  *    plus the period the h1 uses -- never a number computed from the list
@@ -71,14 +73,15 @@ export function Outline({
 }) {
   const Link = useLinkProvider();
   const baseurl = useBaseurl();
-  const { headings } = useHeaders('main h2, main h3', 3);
+  // h2 to h4, the depths the Sphinx panel lists. `maxdepth` stays 3 because
+  // upstream renumbers levels from the shallowest heading on the page and keeps
+  // `level < maxdepth + 1`: with an h2 present, an h4 is level 3.
+  const { headings } = useHeaders('main h2, main h3, main h4', 3);
   const currentId = useActiveHeading(headings);
   const tree = nest(headings);
-  // The current item's own sub-list, and every ancestor of the current item,
-  // are expanded; nothing else is.
-  const expandedId = tree.find(
-    (branch) => branch.id === currentId || branch.children.some((c) => c.id === currentId)
-  )?.id;
+  // The current entry's own sub-list and those of all its ancestors are open;
+  // nothing else is.
+  const open = openBranches(tree, currentId);
   return (
     <div className={classNames('relative self-start', containerClassName)}>
       <nav
@@ -88,27 +91,7 @@ export function Outline({
         {headings.length > 0 && (
           <>
             <p className="qe-outline__title">On this page</p>
-            <ul className="qe-outline__list">
-              {tree.map((branch) => (
-                <li
-                  key={`outline-li-${branch.id}`}
-                  className={classNames({
-                    'qe-outline__expanded': branch.children.length > 0 && branch.id === expandedId,
-                  })}
-                >
-                  <Entry heading={branch} currentId={currentId} Link={Link} />
-                  {branch.children.length > 0 && (
-                    <ul>
-                      {branch.children.map((child) => (
-                        <li key={`outline-li-${child.id}`} className="qe-outline__sub">
-                          <Entry heading={child} currentId={currentId} Link={Link} />
-                        </li>
-                      ))}
-                    </ul>
-                  )}
-                </li>
-              ))}
-            </ul>
+            <Branches nodes={tree} depth={0} currentId={currentId} open={open} Link={Link} />
           </>
         )}
         <div className="qe-outline__logo">
@@ -134,18 +117,88 @@ export function Outline({
 }
 
 type OutlineHeading = { id: string; title: string; level: number; element: HTMLElement };
-type Branch = OutlineHeading & { children: OutlineHeading[] };
+type Branch = OutlineHeading & { children: Branch[] };
 
-/** h3s under the h2 before them; a leading h3 with no h2 starts its own branch. */
+/**
+ * Nests each heading under the nearest preceding shallower one, to any depth.
+ * A heading with nothing shallower before it starts its own branch, so a page
+ * whose first heading is an h3 still renders.
+ */
 function nest(headings: OutlineHeading[]): Branch[] {
   const tree: Branch[] = [];
+  const ancestors: Branch[] = [];
   for (const h of headings) {
-    if (h.level > 1 && tree.length > 0) tree[tree.length - 1].children.push(h);
-    else tree.push({ ...h, children: [] });
+    const branch: Branch = { ...h, children: [] };
+    while (ancestors.length > 0 && ancestors[ancestors.length - 1].level >= h.level) {
+      ancestors.pop();
+    }
+    if (ancestors.length > 0) ancestors[ancestors.length - 1].children.push(branch);
+    else tree.push(branch);
+    ancestors.push(branch);
   }
   return tree;
 }
 
+/**
+ * The ids whose sub-lists the panel opens: the current entry's and every
+ * ancestor's, which is the rule the Sphinx panel's scrollspy applies at any
+ * depth. Marking is separate -- only the current entry itself is marked.
+ */
+function openBranches(tree: Branch[], currentId?: string): Set<string> {
+  const open = new Set<string>();
+  if (!currentId) return open;
+  const walk = (nodes: Branch[], ancestors: string[]): boolean =>
+    nodes.some((node) => {
+      if (node.id === currentId) {
+        for (const id of [...ancestors, node.id]) open.add(id);
+        return true;
+      }
+      return walk(node.children, [...ancestors, node.id]);
+    });
+  walk(tree, []);
+  return open;
+}
+
+/** One level of the outline; renders its children recursively. */
+function Branches({
+  nodes,
+  depth,
+  currentId,
+  open,
+  Link,
+}: {
+  nodes: Branch[];
+  depth: number;
+  currentId?: string;
+  open: Set<string>;
+  Link: ReturnType<typeof useLinkProvider>;
+}) {
+  return (
+    <ul className={depth === 0 ? 'qe-outline__list' : undefined}>
+      {nodes.map((node) => (
+        <li
+          key={`outline-li-${node.id}`}
+          className={classNames({
+            'qe-outline__sub': depth > 0,
+            'qe-outline__expanded': node.children.length > 0 && open.has(node.id),
+          })}
+        >
+          <Entry heading={node} currentId={currentId} Link={Link} />
+          {node.children.length > 0 && (
+            <Branches
+              nodes={node.children}
+              depth={depth + 1}
+              currentId={currentId}
+              open={open}
+              Link={Link}
+            />
+          )}
+        </li>
+      ))}
+    </ul>
+  );
+}
+
 function Entry({
   heading,
   currentId,
diff --git a/docs/layout.md b/docs/layout.md
index e8c0f80e1..298f0b560 100644
--- a/docs/layout.md
+++ b/docs/layout.md
@@ -21,15 +21,19 @@ it and its toggle.
 ## "On this page"
 
 The right-hand outline is pinned (`position: fixed` in the margin column),
-lists h2 and h3 headings with their section numbers when numbering is on, and
-marks the section being read in QuantEcon blue, bold, with an inset rule
-(`aria-current="location"`). The rule is the Sphinx scrollspy's: a section is
-current once its heading has passed 120px from the top, and the last section is
-current at the bottom of the page. Past the viewport height the panel scrolls
-internally behind a fade. Subsections collapse to the current branch as the
-Sphinx panel does under `contents_autoexpand`: only the sections show until
-you scroll into one, its subsections then expand, and the parent of a current
-subsection is expanded but not marked.
+lists **h2, h3 and h4** headings with their section numbers when numbering is
+on, and marks the section being read in QuantEcon blue, bold, with an inset
+rule (`aria-current="location"`). Each level is indented one step further than
+the one above it. h5 and deeper are not listed. The rule is the Sphinx
+scrollspy's: a section is current once its heading has passed 120px from the
+top, and the last section is current at the bottom of the page. Past the
+viewport height the panel scrolls internally behind a fade.
+
+Sub-lists collapse to the current branch as the Sphinx panel does under
+`contents_autoexpand`, at every depth: only the sections show until you scroll
+into one, its subsections then expand, and an h3's h4s expand once that h3 or
+one of those h4s is current. Ancestors of the current entry are expanded but
+not marked — exactly one entry is marked at a time.
 
 ## Back to top
 
diff --git a/styles/quantecon.css b/styles/quantecon.css
index 1369f8128..b49ebe873 100644
--- a/styles/quantecon.css
+++ b/styles/quantecon.css
@@ -582,6 +582,13 @@
   .qe-outline__sub a {
     padding-inline-start: 1.75rem; /* 0.75rem entry padding + 1rem indent */
   }
+  /* One more step for the level below (h4 under an h3). Keyed on nesting
+     rather than a depth class, so the step applies wherever the tree goes;
+     at (0,2,1) it outranks the single-level rule above. Logical padding, so
+     a right-to-left edition indents from the right. */
+  .qe-outline__sub .qe-outline__sub a {
+    padding-inline-start: 2.75rem; /* a second 1rem step */
+  }
   .qe-outline__list a:hover {
     color: var(--qe-link-color);
   }
diff --git a/tests/visual/fixture-no-thebe/outline.md b/tests/visual/fixture-no-thebe/outline.md
index 463d676e7..d9bb88a57 100644
--- a/tests/visual/fixture-no-thebe/outline.md
+++ b/tests/visual/fixture-no-thebe/outline.md
@@ -4,7 +4,7 @@ title: Outline page
 
 # Outline page
 
-This page exercises the "On this page" panel: two levels of headings under
+This page exercises the "On this page" panel: three levels of headings under
 project numbering, so entries carry the heading's own enumerator.
 
 ## First section
@@ -15,6 +15,15 @@ Short section; scrolling here should mark it.
 
 A level-three entry, indented in the panel.
 
+#### A level-four subsection
+
+A level-four entry: listed under its h3, indented one step further, and shown
+only while that h3 or one of its own h4s is current.
+
+#### Another level-four subsection
+
+A second one, so the sub-list is a list.
+
 ### Second subsection
 
 Another one.
diff --git a/tests/visual/theme.spec.ts b/tests/visual/theme.spec.ts
index 7e759792d..5186d6bdf 100644
--- a/tests/visual/theme.spec.ts
+++ b/tests/visual/theme.spec.ts
@@ -369,18 +369,23 @@ test.describe("On this page outline", () => {
     await page.goto(`${noThebeBase}/outline`, { waitUntil: "domcontentloaded" });
     await settle(page);
     const entries = nav(page).locator("ul a");
-    // Two h2 + two h3 + one h2 = five entries, with the heading's own
+    // Three h2, two h3 and two h4 = seven entries, each with the heading's own
     // enumerator, never a list-index number.
-    await expect(entries).toHaveCount(5);
+    await expect(entries).toHaveCount(7);
     // (`titles: true` numbers nothing on this fixture's pages, so the
     // enumerators are section-only; the lecture repos get "3.1." from the
     // same span.)
     await expect(entries.nth(0)).toHaveText(/^1\. First section$/);
     await expect(entries.nth(1)).toHaveText(/^1\.1\. First subsection$/);
-    await expect(entries.nth(4)).toHaveText(/^3\. Last section$/);
-    // Autoexpand: at the top of the page only the sections show.
-    const subs = nav(page).locator("li.qe-outline__sub a");
-    await expect(subs).toHaveCount(2);
+    await expect(entries.nth(2)).toHaveText(/^1\.1\.1\. A level-four subsection$/);
+    await expect(entries.nth(6)).toHaveText(/^3\. Last section$/);
+    // Autoexpand: at the top of the page only the sections show. Every entry
+    // below the top level carries the sub class -- two h3s and two h4s. The
+    // child combinator matters: `li.qe-outline__sub a` would match an h4's
+    // anchor through its h3 ancestor's li, so the count would hold even if the
+    // h4's own li lost the class, and the indent rule keys on that class.
+    const subs = nav(page).locator("li.qe-outline__sub > a");
+    await expect(subs).toHaveCount(4);
     await expect(subs.first()).toBeHidden();
     // Scrolling into the first section expands its subsections, indented:
     // the anchors are block-level, so compare their start padding.
@@ -391,8 +396,14 @@ test.describe("On this page outline", () => {
     await expect(subs.first()).toBeVisible();
     const pad = async (i: number) =>
       parseFloat(await entries.nth(i).evaluate((a) => getComputedStyle(a).paddingInlineStart));
+    // One indent step per level: h2 < h3 < h4.
     expect(await pad(1)).toBeGreaterThan(await pad(0));
-    expect(await pad(3)).toBe(await pad(0));
+    expect(await pad(2)).toBeGreaterThan(await pad(1));
+    // ...and a later h2 is back at the top level's indent.
+    expect(await pad(5)).toBe(await pad(0));
+    // The h4s stay closed while only their h2 is current: an h3's sub-list
+    // opens when that h3, or one of its own h4s, is current.
+    await expect(entries.nth(2)).toBeHidden();
     // Pinned: the panel's top is the same before and after a long scroll.
     const top = async () => Math.round((await nav(page).boundingBox())!.y);
     const before = await top();
@@ -420,17 +431,37 @@ test.describe("On this page outline", () => {
     await scrollTo("second-section", 100);
     await expect(current(page)).toHaveAttribute("href", /#second-section$/);
     await expect(current(page)).toHaveCSS("font-weight", "600");
-    // ...and the first section's sub-list has collapsed again.
+    // ...and the first section's sub-list, h4s included, has collapsed again.
     await expect(nav(page).locator("li.qe-outline__sub a").first()).toBeHidden();
+    await expect(nav(page).getByRole("link", { name: /A level-four subsection/ })).toBeHidden();
     // 20px short of the line: the previous section (a subsection) still holds.
     await scrollTo("second-section", 140);
     await expect(current(page)).toHaveAttribute("href", /#second-subsection$/);
     await scrollTo("first-subsection", 100);
-    // A current subsection is marked itself; its parent is expanded, not marked.
+    // A current subsection is marked itself; its ancestor is expanded, not
+    // marked. The subsection's own sub-list opens too, so both it and its
+    // parent carry the expanded class -- the rule is "the current entry and
+    // every ancestor of it".
     await expect(current(page)).toHaveAttribute("href", /#first-subsection$/);
-    const parent = nav(page).locator("li.qe-outline__expanded > a");
-    await expect(parent).toHaveAttribute("href", /#first-section$/);
-    await expect(parent).not.toHaveAttribute("aria-current", "location");
+    const expanded = nav(page).locator("li.qe-outline__expanded > a");
+    await expect(expanded).toHaveCount(2);
+    await expect(expanded.nth(0)).toHaveAttribute("href", /#first-section$/);
+    await expect(expanded.nth(1)).toHaveAttribute("href", /#first-subsection$/);
+    await expect(expanded.nth(0)).not.toHaveAttribute("aria-current", "location");
+    // Its h4s are now visible.
+    await expect(nav(page).getByRole("link", { name: /A level-four subsection/ })).toBeVisible();
+
+    // A current h4 is marked itself; its h3 and h2 are expanded, not marked.
+    await scrollTo("a-level-four-subsection", 100);
+    await expect(current(page)).toHaveAttribute("href", /#a-level-four-subsection$/);
+    await expect(current(page)).toHaveCount(1);
+    const openNow = nav(page).locator("li.qe-outline__expanded > a");
+    await expect(openNow).toHaveCount(2);
+    await expect(openNow.nth(0)).toHaveAttribute("href", /#first-section$/);
+    await expect(openNow.nth(1)).toHaveAttribute("href", /#first-subsection$/);
+    for (const i of [0, 1]) {
+      await expect(openNow.nth(i)).not.toHaveAttribute("aria-current", "location");
+    }
     // The last section is too short to reach the activation window; the
     // bottom-of-page rule marks it.
     await page.evaluate(() => window.scrollTo(0, document.documentElement.scrollHeight));