I don't think it's mandatory. Earlier they had said Beta 4 (and even Beta 3?) was acceptable for submissions even for launch.
Personally, I would *not* consider leaving things on Beta 4 for one minute, given how many problems it had. Also, you've got time now to rebuild with Gold and re-test... is it not far better to do that now before you're in the position of updating a "live" app with many users, after launch? Unless you have no plans ever to update your app again... if that's the case, just leave it alone if it works.