To the first part of your question, you need to complete one full set of licensing exams in order to get a license. I don't think (but don't know for certain), that it matters which one. And it's unlikely to be program dependent, more likely state licensing board dependent. So if you've taken the other 3 exams in USMLE (as well as COMLEX), just pick one. Preferably the cheaper or easier one.
To the second part of your question. What you should be buying to bridge M4 to PGY1 is a few cases of beer and some time on a beach/mountain/lake/trail. Not a Q Bank.