This gun is used by the SOS (Special Operations of the Supernatural) - a government agency that features in my fanfic.
I made this gun used by the SOS because it's a good gun. Being a Carbine, it has a higher range than the MP5A3. It has an effective range of 500m.
It also has many attachments that can be used. This means that you can swap them to suit any environment. You could even, especially in an urban battlefield, attach a sniper scope and switch it to semi-automatic and shoot people like that.
Alike to the M16, the M4 can have many attachments fitted on to it. An example of these are foregrips (like handles - allow you to shoot a lot more accurately and it's easier to steady), numerous sights and scopes, bipods (used to steady the gun against surfaces or the ground - mostly used for precision-shooting), the M203 or M320 grenade launchers (good for knocking down doors - or buildings), the M26 MASS (it fires a shotgun round and is situated underneath the barrel, like the grenade launchers). The M26 MASS is good for close-quarters combat, again particulary in urban environments.
The M4 Carbine is used by many of the U.S. Special Forces.
It fires 5.56x45mm NATO bullets with a 30 round magazine. It can fire between 700 and 950 rounds per minute and spits them out at about 884m/s.
The M4 Carbine has been used in many films, such as Air Force One, Black Hawk Down, X-Men 2, The Hulk, Terminator 3, 2 Fast 2 Furious, Bad Boys II, Batman Begins, Transporter 2, Divergence, Mission Impossible III, Ghost Rider, Spiderman 3, one of the Die Hard movies, 28 Weeks Later, Transformers, Cloverfield, The Incredible Hulk, The Dark Knight, Tropic Thunder, Fast and Furious, G.I. Joe - Rise of Cobra, Ironman 2, The A-Team, Knight and Day, Inception, Salt, Battle: Los Angeles, Thor, Tower Heist, Battleship, The Avengers and the Amazing Spiderman.
Also many video games, too. Battlefield 2, Grand Theft Auto IV, Saints Row 2, Call of Duty Modern Warfare 2, Just Cause 2, Battlefield Bad Company 2, Homefront, Battlefield 3, Call of Duty Modern Warfare 3, Saints Row The Third and... I can't be bothered to name the rest. There's lots.