The M4 Carbine is essentially a smaller and lighter version of the M16A2 assault rifle. It is so similar that the two weapons share 80% of their parts with each other. The original M4 has selective fire options of semi-automatic and three-round burst, much like the M16A2. The M4A1, and all models after it, have the three-round burst replaced with a fully-automatic fire mode.
The M4 and its variants all fire the 5.56x45mm NATO round and are, like most rifles, gas-operated, air-cooled, magazine-fed, selective fire weapons.
As with most carbines the M4 is much shorter, lighter and easier to carry that a full-length rifle. While this makes it ideal for vehicle crews it also proves its worth in CQC (close-quaters-combat) scenarios and airborne and special operations missions. The M4 family is the preferred weapon by USSOCOM, the Navy Seals and the Green Berets. The Australian SAS has also fielded the M4 series and the weapon is used by law-enforcement agencies around the globe.
M4A1 - Fully automatic, intended for special operations
SOPMOD - Features a RIS (rail-interface-system) that can accommodate many different Reflex-sights, grips, LAMs (laser aiming modules), tactical lights and the M203 40mm grenade launcher. SOPMOD II is also in development.
The M4 series has been popular in video games for some time; partly because it is a famous carbine used heavily by US forces but also because the SOPMOD variant is heavily customizable. This has been heavily exploited in games such as Call of Duty 4: Modern Warfare and Metal Gear Solid 4: Guns of the Patriots. In the former of these two, the player starts many levels with differently customized M4s as both the British SAS and the USMC. In the latter of the two, the protagonist gets his hands on an "M4 Custom" early on in the game and can customize it with different accessories as he progresses through the game.