Good summary here...
https://www.cameraquest.com/mguide.htm
The main thing is that the original M4 is more adjustable internally (same construction philosophy as the M3 and M2), while the later ones were made with simpler parts that have to be in tolerance - if they wear, they need replaced, while the adjustable ones can be adjusted back into tolerance. And the adjustable ones can arguably be tuned more finely. Could be a factor when buying an old camera - a CLA should need less replacement parts if it's the more adjustable type of construction.
Don't know why you fancy an M4 in particular, but the M2 has framelines for 35, 50 and 90mm, so might be an option. On the other hand, it has a knob rewind rather than a crank, and the film counter is a rotating disc that you reset manually. The other main difference is film loading is done by removing the take-up spool (but no need to trim the film leader like on Barnack Leicas). I chose an M2 for its all-metal construction and the fact that the 35mm frameline doesn't have a 135mm frameline in the middle. It's also been said that the angled rewind cranks are more at risk of breaking, while the knobs basically don't break. In practice, the supposed cons of the M2 have never been an issue for me - reloading isn't exactly
slow, just a bit slow
er. Just a thought, which may or may not be relevant.