The M4 is a short barrel carbine.
The main difference is the length of the barrel and the gas tube. The Marines still use the M16A4, which has the longer, heavier barrel, and fires 3-round burst instead of full auto. The longer barrel also has the advantage of giving you a better sight radius. Rifles with the longer gas system are considered to be more reliable as well.
the longer barrel keps the 5.56 mm lethal at longer ranges as well.
That was my main assumption – although there might be some internal differences I might not have heard about.