I was frustrated by this, too. Here’s my workaround:
Copy M5 to a separate drive— in my case, I have a partition with Snow Leopard as David Roth Weiss recommends.
REMOVE M5 entirely from the system drive. Open FCP and check that it roundtrips with M4 (it should as long as you haven’t deleted M4).
Check that you can open M5 from the other drive.
Now you will be able to roundtrip between FCP and M4 and use M5 as a standalone.
Adam Grossman
Good Dog Media