John Koehrer pretty much sums it up. I much prefer the rewind knob of the M2 to the angled crank rewind of the M4. Some prefer the crank. The M2 has only three framelines, all separate -- 35mm, 50mm and 90mm -- which to me is ideal. The M4 adds a 135 frameline, which to me is useless. YMMV. The M4 tends to be more money. I don't mind the M2's loading system; I find that it takes a little more time, but loads perfectly the first time, so it evens out for me in the end.
Originally Posted by Andrey
I really think, at the risk of repeating myself, that they are all good. You can't go wrong with any of them. Find one that you can examine, that is in acceptable condition and goes for a reasonable price. Or buy from a very reputable place like KEH that grades the cameras fairly and takes returns.
One tiny correction to what someone above said: Leicas with .72 viewfinders do have 28mm framelines. I have seen them in the M6, the M7 and the MP. I believe the M4-P also had a 28mm frameline.