I was going to have to disagree with first answer also...
An 'A4' is a military aircraft.
An AR-15 is a civilian version of a military issued rife.
Later Variations of the M-16 were designated as 'M-16A1', 'M-16A2', 'M-16A3', M-16A4', ect.
What the OP was probably trying to ask was about the difference between the current production versions of the AR-15 and the military issued M-16A4,
The OP was trying to ask about the 'M-4' military carbine.
Anyway, now I've confused myself!