[Karagin]

M4 is the 3 round burst rifle/carbine, the M4A1 is the one that goes back to the M16 and M16A1 with full auto in place of 3 round burst.

Second, the M4 with Optics is the standard-issue RIFLE for ALL US Army units, with the expectation of cooks in some units. So everyone from infantry to admin has in M4 or M4A1 in combat.

Third, the shorter ranged Carbines are things like CAR-15 and M4 Commando, those aren't issued to line or support units, those are SPEC OPS toys.