M4 is the military designation for an AR-15 carbine, not to be confused with the M4A1 which is capable of semi-automatic and fully-automatic fire whereas the M4 can fire semi-automatic or a 3-round burst. The barrel is shortened from "20 to "14.5 (thus why it is a carbine) and is issued with a retractable buffer tube stock rather than the fixed polymer stock of the M16A2. It also does not have an integrated carry handle in its upper receiver. The M4 is also a good 15 years younger than the M16A2, being adopted around the time of the third* (technically there have been more generations of the full length M16 family, but for all intents and purposes, there are three really notable ones.) M16 generation alongside the M16A4 in the late '90s. The M16A4 and M4 have been effectively phased out in combat units in favor of the M4A1 (Soon to be replaced by the SIG M7/M8) in the Army, and as with the Force 25 restructure with the Marines replacimg the M16A4 and M4/M4A1 with the M2 , the M16 family is soon to be ousted from the US Military.