diff --git a/CHANGES.md b/CHANGES.md
index 0c48a637851..800eafd6adf 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -151,6 +151,9 @@ Unreleased
   `INSIDE_EMACS` in order to improve reproducibility (#4680,
   @jeremiedimino)
 
+- Fix `root_module` when used in public libraries (#4685, fixes #4684,
+  @rgrinberg, @CraigFe)
+
 2.9.0 (unreleased)
 ------------------
 
diff --git a/src/dune_rules/dune_file.ml b/src/dune_rules/dune_file.ml
index 1ecfc84d6d4..85a9af05c8b 100644
--- a/src/dune_rules/dune_file.ml
+++ b/src/dune_rules/dune_file.ml
@@ -789,7 +789,12 @@ module Library = struct
         false
     in
     Obj_dir.make_lib ~dir
-      ~has_private_modules:(t.private_modules <> None)
+      ~has_private_modules:
+        ((* TODO instead of this fragile approximation, we should be looking at
+            [Modules.t] and deciding. Unfortunately, [Obj_dir.t] is currently
+            used in some places where [Modules.t] is not yet constructed. *)
+         t.private_modules <> None
+        || t.buildable.root_module <> None)
       ~private_lib (snd t.name)
 
   let main_module_name t : Lib_info.Main_module_name.t =
diff --git a/test/blackbox-tests/test-cases/github4684.t b/test/blackbox-tests/test-cases/github4684.t
index 4cf54ec6633..99935e1df9f 100644
--- a/test/blackbox-tests/test-cases/github4684.t
+++ b/test/blackbox-tests/test-cases/github4684.t
@@ -12,80 +12,4 @@ build failure:
   > EOF
 
   $ dune build
-  Internal error, please report upstream including the contents of _build/log.
-  Description:
-    ("External.cm_dir",
-    { t =
-        { public_dir = In_build_dir "default"
-        ; private_dir = None
-        ; public_cmi_dir = None
-        }
-    })
-  Raised at file "otherlibs/stdune-unstable/code_error.ml", line 11, characters
-    30-62
-  Called from file "src/dune_rules/install_rules.ml", line 148, characters
-    19-69
-  Called from file "src/dune_rules/install_rules.ml" (inlined), line 165,
-    characters 23-31
-  Called from file "src/dune_rules/install_rules.ml", line 180, characters
-    49-63
-  Called from file "list.ml", line 103, characters 22-25
-  Called from file "otherlibs/stdune-unstable/list.ml", line 5, characters
-    19-33
-  Called from file "list.ml", line 103, characters 22-25
-  Called from file "otherlibs/stdune-unstable/list.ml" (inlined), line 5,
-    characters 19-33
-  Called from file "otherlibs/stdune-unstable/list.ml", line 40, characters
-    29-39
-  Called from file "src/dune_rules/install_rules.ml", line 155, characters
-    6-1023
-  Called from file "src/fiber/fiber.ml", line 182, characters 9-14
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 472, characters 10-22
-  Called from file "src/fiber/fiber.ml", line 127, characters 10-17
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  Re-raised at file "otherlibs/stdune-unstable/exn.ml", line 36, characters
-    27-56
-  Called from file "src/fiber/fiber.ml", line 204, characters 8-13
-  -> required by ("stanzas-to-entries", "default")
-  -> required by ("<unnamed>", ())
-  -> required by ("<unnamed>", ())
-  -> required by ("<unnamed>", ())
-  -> required by ("<unnamed>", ())
-  -> required by ("load-dir", In_build_dir "default")
-  -> required by ("build-alias", { dir = "default"; name = "default" })
-  
-  I must not crash.  Uncertainty is the mind-killer. Exceptions are the
-  little-death that brings total obliteration.  I will fully express my cases. 
-  Execution will pass over me and through me.  And when it has gone past, I
-  will unwind the stack along its path.  Where the cases are handled there will
-  be nothing.  Only I will remain.
-  [1]