No big difference.
The M4 has 35/50/90/135 frame lines as does the M4-2, The -2 also has a hot shoe.
I personally would pass on anything more recent than the 4 because I don't see the need for a meter or ttl flash.
I really think the M2 is the best of 'em all... 35/50/90 frame lines, if you can find one with the original rangefinder it has a depth of field indicator in the VF, only for f5.6 & f16. The 4's are a bit easier to load because you don't need to pull the take up spool out to load the camera and they also have a crank on the rewind knob.