[pickle] Fix value of pickle.DEFAULT_PROTOCOL for Python <3.14 (#15446)

stdlib/pickle.pyi

@@ -1,3 +1,4 @@
+import sys
 from _pickle import (
     PickleError as PickleError,
     Pickler as Pickler,
@@ -103,7 +104,10 @@ __all__ = [
 ]
 
 HIGHEST_PROTOCOL: Final = 5
-DEFAULT_PROTOCOL: Final = 5
+if sys.version_info >= (3, 14):
+    DEFAULT_PROTOCOL: Final = 5
+else:
+    DEFAULT_PROTOCOL: Final = 4
 
 bytes_types: tuple[type[Any], ...]  # undocumented