AK-47 is a weapon for standard troops designed to open big holes to the enemy and put him down once and for good. It is also much heavier thus not recomended for commando squads. It's job is to kill drugged sholdiers that charge against you (cause 5.56mm ammunition must hit very vital body parts to kill and this does not happen instantly if it ain't a headshot) or just kill anything.
The M4A1 or the M16A3 are designed for commandos. These weapons have to be lighter so that the elite troops that use them can travel further on foot. The M4 is shorter than the M16 and is best choise for cityfight. The 5.56 philosophy is that you wound a man so that his comrades will try to resque him and will stop fire in order to do it. M4 and M16 (and M203 launcher) have special amunition so that they don't leave but few dirt in the internal part of the weapon plus its black metal (not painted but black so that scratches do nothing on it) make them reliable for longer firefights and nightfights.