It is very simple, the M2 has three frames,35/50/90 and as I rarely use 135 the added frame in the M4 is irritating. The M4 also has the angled rewind crank and I do not know how many of these I have bent or jammed tight. I also find that I can rarely rewind a whole roll on a M4 sryle rewind without loosing the grip on the "knob". The M2 has a substantial knob that is easy to hold on to.
There is also the fact that I started shooting with M3's and quickly adapted to the M2's because of the 35 frame.
As an added benefit, I can make a Rpaidwinder for the M2 without any modification to the body. The M4 requires either a M6 type intermediate drive or the M4M/Mot type long shaft attchement.
With a couple of M2's, a 21 and a 35/50 lens kit you can do just about anything you can expect from a rangefinder and if you add a small compact 90 you should be all set.
In the end it is the picture that counts and sometimes putting to much complexity between us and the subject slows us down.