diff --git a/backend/migrations/versions/0004_phase4_pdf_open_mode_tsvector.py b/backend/migrations/versions/0004_phase4_pdf_open_mode_tsvector.py
new file mode 100644
index 0000000..f1aab00
--- /dev/null
+++ b/backend/migrations/versions/0004_phase4_pdf_open_mode_tsvector.py
@@ -0,0 +1,85 @@
+"""Phase 4 schema additions: pdf_open_mode column, GIN FTS index, audit-logs bucket.
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-05-25
+
+Changes (in order — see upgrade() for numbered sections):
+  1. Add users.pdf_open_mode column (server_default='in_app')
+  2. Create GIN expression index ix_documents_fts on documents.extracted_text
+  3. Create audit-logs MinIO bucket (gated on MINIO_ENDPOINT env var)
+
+Note on GIN index (T-04-02-01 mitigated):
+  Index is created via raw SQL op.execute() rather than Alembic Index() to prevent
+  Alembic's autogenerate from attempting to recreate the expression index on every
+  `alembic revision --autogenerate` run (Alembic issue #1390). A comment marks it
+  as manually managed.
+
+Note on MinIO bucket creation (T-04-02-02 mitigated):
+  Bucket creation is gated on MINIO_ENDPOINT env var so SQLite test runs are
+  unaffected. Credentials come exclusively from env vars — none are hardcoded.
+  MinIO default bucket policy is private; no public-access policy is set.
+
+Note on downgrade():
+  The GIN index and pdf_open_mode column are reversed. The audit-logs MinIO bucket
+  is NOT deleted — it may contain audit data.
+"""
+from __future__ import annotations
+
+import os
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "0004"
+down_revision = "0003"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # ── 1. Add users.pdf_open_mode column ────────────────────────────────────
+    # batch_alter_table is required for SQLite ALTER TABLE support (transparent
+    # pass-through on PostgreSQL — no behavioral difference on production DB).
+    with op.batch_alter_table("users") as batch_op:
+        batch_op.add_column(
+            sa.Column(
+                "pdf_open_mode",
+                sa.String(),
+                nullable=False,
+                server_default="in_app",
+            )
+        )
+
+    # ── 2. Create GIN expression index on documents.extracted_text ───────────
+    # managed manually — do not autogenerate (Alembic issue #1390)
+    op.execute(
+        "CREATE INDEX ix_documents_fts ON documents "
+        "USING GIN (to_tsvector('english', coalesce(extracted_text, '')))"
+    )
+
+    # ── 3. Create audit-logs MinIO bucket ────────────────────────────────────
+    # Skipped if MINIO_ENDPOINT is not set (SQLite test compatibility — T-04-02-02).
+    if os.environ.get("MINIO_ENDPOINT"):
+        from minio import Minio  # deferred import — only needed when MinIO is configured
+
+        client = Minio(
+            os.environ.get("MINIO_ENDPOINT", "minio:9000"),
+            access_key=os.environ.get("MINIO_ACCESS_KEY", ""),
+            secret_key=os.environ.get("MINIO_SECRET_KEY", ""),
+            secure=False,
+        )
+        if not client.bucket_exists("audit-logs"):
+            client.make_bucket("audit-logs")
+
+
+def downgrade() -> None:
+    # ── 1. Drop GIN expression index ─────────────────────────────────────────
+    op.execute("DROP INDEX IF EXISTS ix_documents_fts")
+
+    # ── 2. Drop users.pdf_open_mode column ───────────────────────────────────
+    with op.batch_alter_table("users") as batch_op:
+        batch_op.drop_column("pdf_open_mode")
+
+    # MinIO bucket NOT reversed — bucket may contain audit data
diff --git a/backend/storage/minio_backend.py b/backend/storage/minio_backend.py
index 516d060..cc8ba9f 100644
--- a/backend/storage/minio_backend.py
+++ b/backend/storage/minio_backend.py
@@ -85,6 +85,28 @@ class MinIOBackend(StorageBackend):
         )
         return object_key
 
+    async def put_object_raw(
+        self,
+        bucket: str,
+        key: str,
+        data: io.BytesIO,
+        length: int,
+        content_type: str,
+    ) -> None:
+        """Upload bytes to an arbitrary bucket+key (used for audit-logs CSV export).
+
+        Unlike put_object(), does NOT apply the document key schema — the caller
+        supplies the complete key. The main documents bucket is NOT used.
+        """
+        await asyncio.to_thread(
+            self._client.put_object,
+            bucket,
+            key,
+            data,
+            length=length,
+            content_type=content_type,
+        )
+
     async def get_object(self, object_key: str) -> bytes:
         """Fetch object bytes from MinIO by key."""