it depends if the "m3" player has 40+% paints from those M4s
rail isn't useful in less situations than freeze or thunder, but old m2 kits paints make difference...
and have atleast 2 m3s, then get3rd, then you can play with those you have => MU to m3-m4
but dont forget, that its harder to get m4 something, than m3 two, three things:
m4 needs 50% on turret, and then MUing is harder, cuz you have to pay insta cost upgrade while doing it in one day (50% off on MU+speed up), or full price in many days. (cost is nearly same..., and BIG)
some m3s needs one new year/tankmans day/TO birthday sale or variety of 50% on one thing
M4 combo + full MU paint still rockz