I think you just have to do as many as you think will make you feel confident. The way I see it, as long as you don't subtract time from actual studying, you can't take too many practice items. Granted, I haven't taken the thing yet either, so maybe this is all the wrong way to go.
4000 questions actually sounds about right for me too. By the time it comes time for my test date, I'll have done all of Qbank and IV Qbank, the released items, 2 full length simulated exams, and about half of MedRevu.