Same damage, ammo.
M4A1 is pretty expensive ($3100), AK-47 is cheaper ($2500)
M4A1 better accuracy, AK-47 has less acc.
(in a 2D game accuracy doesn't matter as much,
you cover a larger area of bullets with AK-47 and that comes with alot of advantages in a 2D game,
M4A1 can spray small corridors/paths)
so no, the M4A1 is not overpowered AT ALL.
I see no arguements whatsoever, just a bunch of personal experiences you've had being/seeing people killed/killing people with the M4A1. a skilled player can make any weapon look overpowered when he's playing with them.