diff --git a/paperforge/commands/__init__.py b/paperforge/commands/__init__.py
index e6b2d4be..b0cf2ad2 100644
--- a/paperforge/commands/__init__.py
+++ b/paperforge/commands/__init__.py
@@ -17,6 +17,7 @@ _COMMAND_REGISTRY: dict[str, str] = {
     "paper-status": "paperforge.commands.paper_status",
     "agent-context": "paperforge.commands.agent_context",
     "prune": "paperforge.commands.prune",
+    "paper-navigation": "paperforge.commands.paper_navigation",
     "reading-log": "paperforge.commands.reading_log",
 }
 
diff --git a/paperforge/commands/paper_navigation.py b/paperforge/commands/paper_navigation.py
new file mode 100644
index 00000000..1a4c5790
--- /dev/null
+++ b/paperforge/commands/paper_navigation.py
@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+import argparse
+import json
+
+from paperforge import __version__ as PF_VERSION
+from paperforge.core.result import PFResult
+from paperforge.core.io import read_json
+from paperforge.retrieval.structure_tree import summarize_role_index
+
+
+def _resolve_paper_root(vault_path, query: str):
+    """Resolve a paper query to its OCR root directory."""
+    from paperforge.memory.db import get_connection, get_memory_db_path
+    from paperforge.memory.query import lookup_paper
+    from paperforge.worker._utils import pipeline_paths
+
+    db_path = get_memory_db_path(vault_path)
+    if not db_path or not db_path.exists():
+        return None
+    conn = get_connection(db_path, read_only=True)
+    try:
+        entries = lookup_paper(conn, query)
+        if entries:
+            ocr_root = pipeline_paths(vault_path)["ocr"]
+            return ocr_root / entries[0]["zotero_key"]
+    finally:
+        conn.close()
+    return None
+
+
+def run(args: argparse.Namespace) -> int:
+    paper_root = _resolve_paper_root(args.vault_path, args.query)
+    if paper_root is None:
+        data = {"mode": "not_found", "paper_id": args.query, "error": "Paper not found in vault"}
+        if args.json:
+            print(PFResult(ok=False, command="paper-navigation", version=PF_VERSION, data=data).to_json())
+        else:
+            print(json.dumps(data, ensure_ascii=False, indent=2))
+        return 1
+
+    tree_path = paper_root / "index" / "structure-tree.json"
+    if tree_path.exists():
+        tree = read_json(tree_path)
+        payload = {"mode": "structure_tree", "paper_id": tree.get("paper_id", ""), "nodes": tree.get("nodes", [])}
+    else:
+        role_index_path = paper_root / "index" / "role-index.json"
+        if role_index_path.exists():
+            role_index = read_json(role_index_path)
+            summary = summarize_role_index(role_index)
+            payload = {"mode": "role_index_summary", "paper_id": args.query, "summary": summary}
+        else:
+            payload = {"mode": "no_index", "paper_id": args.query}
+
+    result = PFResult(ok=True, command="paper-navigation", version=PF_VERSION, data=payload)
+    if args.json:
+        print(result.to_json())
+    else:
+        print(json.dumps(payload, ensure_ascii=False, indent=2))
+    return 0
diff --git a/paperforge/retrieval/__init__.py b/paperforge/retrieval/__init__.py
new file mode 100644
index 00000000..a2576981
--- /dev/null
+++ b/paperforge/retrieval/__init__.py
@@ -0,0 +1,3 @@
+"""paperforge.retrieval — retrieval substrate for paper navigation and search."""
+
+from __future__ import annotations
diff --git a/paperforge/retrieval/gateway.py b/paperforge/retrieval/gateway.py
new file mode 100644
index 00000000..191211e7
--- /dev/null
+++ b/paperforge/retrieval/gateway.py
@@ -0,0 +1,67 @@
+"""paperforge.retrieval.gateway — Layer 4 gateway command routing.
+
+Provides the core `route_gateway()` function that all gateway commands
+route through. Currently delegates to existing capabilities; later tasks
+will upgrade routing to use real Layer 4 artifacts.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from paperforge import __version__ as PF_VERSION
+from paperforge.core.result import PFResult
+from paperforge.query_planning import build_query_plan, enrich_query_plan_with_runtime
+
+# Map gateway intent names to build_query_plan intent strings
+INTENTS: dict[str, str] = {
+    "paper-lookup": "known-paper",
+    "content-discovery": "content",
+    "paper-navigation": "known-paper",
+    "scoped-fetch": "known-paper",
+}
+
+
+def route_gateway(
+    vault: Path,
+    intent: str,
+    query: str,
+    *,
+    json_mode: bool,
+    limit: int = 5,
+) -> PFResult:
+    """Route a gateway command through the query planning pipeline.
+
+    Parameters
+    ----------
+    vault : Path
+        PaperForge vault root.
+    intent : str
+        Gateway intent name (e.g. ``"paper-lookup"``).
+    query : str
+        Free-text or structured query.
+    json_mode : bool
+        Whether the caller expects JSON output.
+    limit : int, optional
+        Maximum result count (default 5).
+
+    Returns
+    -------
+    PFResult
+        Result packet with the route plan and intent metadata.
+    """
+    plan = enrich_query_plan_with_runtime(
+        build_query_plan(query, INTENTS[intent]),
+        vault,
+    )
+    return PFResult(
+        ok=True,
+        command=intent,
+        version=PF_VERSION,
+        data={
+            "intent": intent,
+            "query": query,
+            "route_plan": plan,
+            "limit": limit,
+        },
+    )
diff --git a/paperforge/retrieval/structure_tree.py b/paperforge/retrieval/structure_tree.py
new file mode 100644
index 00000000..8bf184da
--- /dev/null
+++ b/paperforge/retrieval/structure_tree.py
@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from paperforge.core.io import write_json
+
+
+def build_structure_tree(structured_blocks: list[dict[str, Any]]) -> dict[str, Any]:
+    nodes: list[dict[str, Any]] = []
+    current_section: dict[str, Any] | None = None
+    for block in structured_blocks:
+        role = block.get("role")
+        text = str(block.get("text", "")).strip()
+        if role in {"section_heading", "subsection_heading", "introduction_heading", "abstract_heading"} and text:
+            current_section = {
+                "node_id": f"sec:{block.get('block_id')}",
+                "kind": "section",
+                "title": text,
+                "level": 1 if role != "subsection_heading" else 2,
+                "section_path": [text] if role != "subsection_heading" else [nodes[-1]["title"], text] if nodes else [text],
+                "page_span": [block.get("page", 0), block.get("page", 0)],
+                "block_span": [[block.get("page", 0), block.get("block_id", "")]],
+                "children": [],
+                "objects": [],
+            }
+            nodes.append(current_section)
+        elif current_section is not None:
+            current_section["page_span"][1] = block.get("page", current_section["page_span"][1])
+            current_section["block_span"].append([block.get("page", 0), block.get("block_id", "")])
+    return {"paper_id": structured_blocks[0].get("paper_id", "") if structured_blocks else "", "nodes": nodes}
+
+
+def write_structure_tree(index_root: Path, tree: dict[str, Any]) -> None:
+    index_root.mkdir(parents=True, exist_ok=True)
+    write_json(index_root / "structure-tree.json", tree)
+
+
+def summarize_role_index(role_index: dict[str, Any]) -> dict[str, Any]:
+    role_counts: dict[str, int] = {}
+    for key, entries in role_index.items():
+        if isinstance(entries, list):
+            role_counts[key] = len(entries)
+    return {"role_counts": role_counts}
diff --git a/paperforge/worker/ocr.py b/paperforge/worker/ocr.py
index 89f64539..9e9ff2e0 100644
--- a/paperforge/worker/ocr.py
+++ b/paperforge/worker/ocr.py
@@ -2029,6 +2029,11 @@ def postprocess_ocr_result(vault: Path, key: str, all_results: list[dict]) -> tu
         resolved_metadata=resolved,
     )
     write_role_index(ocr_root / "index", role_indexes)
+    # --- Phase 6: structure tree ---
+    from paperforge.retrieval.structure_tree import build_structure_tree, write_structure_tree
+
+    structure_tree = build_structure_tree(structured)
+    write_structure_tree(ocr_root / "index", structure_tree)
 
     # Update meta.json with version payloads
     ocr_model = meta.get("ocr_model", meta.get("ocr_provider", "PaddleOCR"))
diff --git a/paperforge/worker/ocr_rebuild.py b/paperforge/worker/ocr_rebuild.py
index 1d521b77..291433e8 100644
--- a/paperforge/worker/ocr_rebuild.py
+++ b/paperforge/worker/ocr_rebuild.py
@@ -398,6 +398,11 @@ def run_derived_rebuild_for_keys(vault: Path, keys: list[str], progress_bar=None
             resolved_metadata=resolved,
         )
         write_role_index(paper_root / "index", role_indexes)
+        # Rebuild structure tree
+        from paperforge.retrieval.structure_tree import build_structure_tree, write_structure_tree
+
+        structure_tree = build_structure_tree(structured)
+        write_structure_tree(paper_root / "index", structure_tree)
 
         # Update version state in meta.json
         meta = read_json(artifacts.meta_json) if artifacts.meta_json.exists() else {}
diff --git a/tests/test_layer4_structure_tree.py b/tests/test_layer4_structure_tree.py
new file mode 100644
index 00000000..85222d95
--- /dev/null
+++ b/tests/test_layer4_structure_tree.py
@@ -0,0 +1,114 @@
+"""Tests for Layer 4 structure tree builder and paper navigation."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import patch
+
+from paperforge.retrieval.structure_tree import (
+    build_structure_tree,
+    summarize_role_index,
+    write_structure_tree,
+)
+
+
+def test_build_structure_tree_creates_section_nodes_from_headings():
+    structured_blocks = [
+        {"paper_id": "ABCD1234", "page": 1, "block_id": "b1", "role": "section_heading", "text": "Methods"},
+        {"paper_id": "ABCD1234", "page": 1, "block_id": "b2", "role": "body_paragraph", "text": "We recruited 30 patients."},
+    ]
+    tree = build_structure_tree(structured_blocks)
+    assert tree["paper_id"] == "ABCD1234"
+    assert len(tree["nodes"]) == 1
+    assert tree["nodes"][0]["title"] == "Methods"
+    assert tree["nodes"][0]["section_path"] == ["Methods"]
+    assert tree["nodes"][0]["level"] == 1
+    assert tree["nodes"][0]["page_span"] == [1, 1]
+
+
+def test_build_structure_tree_handles_subsection():
+    structured_blocks = [
+        {"paper_id": "ABCD1234", "page": 1, "block_id": "b1", "role": "section_heading", "text": "Methods"},
+        {"paper_id": "ABCD1234", "page": 1, "block_id": "b2", "role": "body_paragraph", "text": "We recruited 30 patients."},
+        {"paper_id": "ABCD1234", "page": 2, "block_id": "b3", "role": "subsection_heading", "text": "Statistical Analysis"},
+        {"paper_id": "ABCD1234", "page": 2, "block_id": "b4", "role": "body_paragraph", "text": "We used t-tests."},
+    ]
+    tree = build_structure_tree(structured_blocks)
+    assert tree["paper_id"] == "ABCD1234"
+    assert len(tree["nodes"]) == 2
+    assert tree["nodes"][1]["title"] == "Statistical Analysis"
+    assert tree["nodes"][1]["level"] == 2
+    assert tree["nodes"][1]["section_path"] == ["Methods", "Statistical Analysis"]
+
+
+def test_build_structure_tree_extends_page_span():
+    structured_blocks = [
+        {"paper_id": "X", "page": 1, "block_id": "s1", "role": "section_heading", "text": "Introduction"},
+        {"paper_id": "X", "page": 1, "block_id": "p1", "role": "body_paragraph", "text": "Para 1"},
+        {"paper_id": "X", "page": 2, "block_id": "p2", "role": "body_paragraph", "text": "Para 2"},
+        {"paper_id": "X", "page": 3, "block_id": "p3", "role": "body_paragraph", "text": "Para 3"},
+    ]
+    tree = build_structure_tree(structured_blocks)
+    assert tree["nodes"][0]["page_span"] == [1, 3]
+
+
+def test_build_structure_tree_handles_introduction_and_abstract_headings():
+    blocks = [
+        {"paper_id": "Y", "page": 1, "block_id": "a1", "role": "abstract_heading", "text": "Abstract"},
+        {"paper_id": "Y", "page": 1, "block_id": "b1", "role": "abstract_body", "text": "Summary here."},
+        {"paper_id": "Y", "page": 1, "block_id": "i1", "role": "introduction_heading", "text": "Introduction"},
+        {"paper_id": "Y", "page": 2, "block_id": "b2", "role": "body_paragraph", "text": "Background."},
+    ]
+    tree = build_structure_tree(blocks)
+    assert len(tree["nodes"]) == 2
+    assert tree["nodes"][0]["title"] == "Abstract"
+    assert tree["nodes"][1]["title"] == "Introduction"
+    assert tree["nodes"][0]["kind"] == "section"
+
+
+def test_build_structure_tree_ignores_heading_roles_with_empty_text():
+    blocks = [
+        {"paper_id": "Z", "page": 1, "block_id": "h1", "role": "section_heading", "text": ""},
+        {"paper_id": "Z", "page": 1, "block_id": "b1", "role": "body_paragraph", "text": "Some text."},
+    ]
+    tree = build_structure_tree(blocks)
+    assert len(tree["nodes"]) == 0
+
+
+def test_build_structure_tree_empty_input():
+    tree = build_structure_tree([])
+    assert tree["paper_id"] == ""
+    assert tree["nodes"] == []
+
+
+def test_write_structure_tree_creates_file(tmp_path):
+    tree = {"paper_id": "X", "nodes": []}
+    write_structure_tree(tmp_path, tree)
+    target = tmp_path / "structure-tree.json"
+    assert target.exists()
+    import json
+    assert json.loads(target.read_text(encoding="utf-8")) == tree
+
+
+def test_summarize_role_index_counts_roles():
+    role_index = {
+        "headings": [{"paper_id": "X"}, {"paper_id": "X"}],
+        "body_paragraphs": [{"paper_id": "X"} for _ in range(5)],
+        "figure_captions": [{"paper_id": "X"}],
+    }
+    summary = summarize_role_index(role_index)
+    assert summary["role_counts"]["headings"] == 2
+    assert summary["role_counts"]["body_paragraphs"] == 5
+    assert summary["role_counts"]["figure_captions"] == 1
+
+
+def test_build_structure_tree_block_span_accumulates():
+    blocks = [
+        {"paper_id": "P", "page": 1, "block_id": "sec1", "role": "section_heading", "text": "Results"},
+        {"paper_id": "P", "page": 1, "block_id": "r1", "role": "body_paragraph", "text": "Result A"},
+        {"paper_id": "P", "page": 2, "block_id": "r2", "role": "body_paragraph", "text": "Result B"},
+    ]
+    tree = build_structure_tree(blocks)
+    assert len(tree["nodes"][0]["block_span"]) == 3
+    assert [1, "sec1"] in tree["nodes"][0]["block_span"]
+    assert [2, "r2"] in tree["nodes"][0]["block_span"]