Anyone ever wonder about this?

Everything is the same, except in the U.S. military, every M1 Garand, M1/M1A1 carbine, and any SMGs are replaced by the current standard Army M4, but with iron sights. Snipers, machine gunners, etc. will all keep their historical weapons, but the standard weapon is now the M4 that replaces most weapons.

Germany never really had a chance by that point but just for fun, how much better do you think it would be?

Pros:

-MASSIVE increase in volume of fire
-Able to carry MUCH more ammo
-Easier to transport supplies due to lighter ammo

Cons:

-Obstacle penetration, can't shoot through stuff as well as a 30-06
-I would think an M4 is less accurate than an M1 Garand, especially at the longest ranges, but how often did combat take place at those ranges where it would matter anyway.
-Less powerful at long range
-For interior combat, a dedicated SMG like the Thompson would probably be better than an M4. Unless it's the full auto version I guess then whatever.

Overall I think it would be an absolutely crushing advantage in firefights and logistics.

Or what if the same scenario applied, but for the Germans with their G36, or the Russians with an AK-74? For the Germans I don't think it would matter that much due to their overall situation, but for the Russians with their typical manpower advantage, having that many AK's I think would be amazing.

only standard M4s with the shorty barrels they have now, or will there be Mk12s

I don't keep up with the latest models but a standard barrel (what is it like 16 inch? (no homo)), no shorty/whatever. No Mk12s though that's kind of a special/designated marksman rifle. Snipers still have to keep their Springfields