m16a1
m16a2
m4
m16a4
The prototype army-version, XM16E1, was essentially the same weapon as the M16 with the addition of a forward assist. The M16A1 was the finalized production model. To address issues raised by the XM16E1's testing cycle, a "bird-cage" flash suppressor replaced the XM16E1's three-pronged flash suppressor
The development of the M16A2 rifle was originally requested by the United States Marine Corps as a result of the USMC's combat experiences in Vietnam with the M16 and M16A1.[1] The Marines were the first branch of the US Armed Forces to adopt the M16A2 in the early/mid 1980s with the United States Army following suit in adopting the M16A2 in the late 1980s. Modifications to the M16A2 were more extensive. In addition to the new rifling, the barrel was made with a greater thickness in front of the front sight post to resist bending in the field, allow for cooling between shots, and the stepped-up design allowed use of the M203 grenade launcher to be used with the weapon.[1] The front sight was now a square post with 4 detent positions in order to refine the sight picture. A new adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters to take full advantage of the ballistic characteristics of the new SS109 rounds and to allow windage adjustments without the need of a tool or cartridge.[1] The flash suppressor was again modified, this time to be closed on the bottom so it would not kick up dirt or snow when being fired from the prone position, and acting as a recoil compensator.
The M16A3 was a fully-automatic variant of the M16A2 adopted in small numbers around the time of the introduction of the M16A2, primarily by the U.S. Navy for use by SEAL, SeaBee, and Security units. It features a Safe-Semi-Auto (S-1-F) trigger group like that of the M16A1.
The M16A4, now standard issue for front-line U.S. Marine Corps and some U.S. Army units, replaces the combination fixed carry handle/rear iron sight with a MIL-STD-1913 Picatinny rail, allowing for the rifle to be equipped with a carry handle and/or most military and consumer scopes or sighting systems. Most of the U.S. Marine Corps' M16A4s are equipped with a Knight's Armament Company M5 RAS handguard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached. U.S. Army M16A4s also often feature the KAC M5 RAS. In U.S. Army Field Manuals, M16A4s fitted with the RAS are sometimes referred to as M16A4 MWS or Modular Weapon System. This model retains the 3-round burst mode of the M16A2.
The M4 Carbine was developed from various outgrowths of these designs, including a number of 14.5-inch-barreled A1 style carbines. The XM4 (Colt Model 720) started its trials in the mid-80s, with a 14.5-inch (368mm) barrel. Officially adopted as a replacement for the M3 "Grease Gun" (and the Beretta M9 and M16A2 for select troops) in 1994, it was used with great success in the Balkans and in the War on Terror, including the Afghanistan and Iraq theaters. It is three round burst on the standard carbine, and full auto on the M4A1 Carbine.
Lots more info but these are just some snips from wiki and others.