It’s official. FCP 4.0 and earlier is NOT supported by Leopard. So if you one of the few that are still using this version (why you aren’t on the more stable FCP 4.5 is beyond me) and you are jonzing to upgrade to Leopard…don’t.
Now…UNOFFICIALLY if you are one of the bright ones using FCP 4.5…stick to OS 10.3.9 and QT 6.5. Not sure what APPLE’S stance on this, but from the posts I read, and from experience, this is the most stable combination for FCP 4.5. ALWAYS stick with the most stable version. BACK TO MY MAC and SPACES is cool, but not cool enough to sacrifice a working production machine.