I have a 2a and a 4a (2nd model) and have handled the 4am. They're all very similar. The 2a and 4a are extremely close -- the 2a is smoother and tighter, but was about 50% more expensive. On the other hand, it's also really old. The 4am has a couple of design advantages, like a fixed take-up spool.
I think you'd be happy with any of them if you get one that was finished on a good day, and without light leaks. From what I've heard, Kievs built in the 60's (4a) are the most likely to be well built. Having said that, my 2a from '57 is damn near perfect and buttery smooth -- even if it is really old.