Plastic gears? If we're talking about the same parts, I'm pretty sure the brass gears (M2,3,& 4s) were replaced with steel gears (for motor winder) starting with the M4-2. These sometimes made them seem slightly "rougher" at least at first. All subsequent M's (except perhaps MP) used similar gears
Tom A wrote positively about the M4-2 on RFF
http://www.rangefinderforum.com/forums/showthread.php?t=37655&page=2 :
"The M4-2 is a good camera, but some of the earlier ones had problem with "shutter" bounce. If you pick one up today. it most likely has been fixed. The M4 is a classic, but they are all getting on in age. The advantage of the M2/M4-2/M4P and M6 is that the finders can be fixed and the mechanics are the same.
I have found that on the later M4P's and early M6, apart from having a zinc diecast top-plate ( it doesn't dent, it cracks!), the rewind knob can easily be bent out of shape and it is very difficult to straighten it out. On the M4 and the earlier M4P's it was still brass and more malleable than the cast ones.
They are all good "shooting" cameras and after a CLA (clean,lube and adjust) they should last for a couple of more decades.
Most of the chrome M4's went to non-pro's and they usually took some care of them. The black paint M4's are worn and might need expensive service to replace advance gear. Brass is smooth, but it also wears faster than the later steel gears. The "harshest" M I ever used was a pair of MP's in the 60's. The hardened steel gears felt like they were chrushing rocks! The smoothest M i have is a dead heat between a couple of M2's and the latest version, the MP. Both will click and advance like stroking silk!"