The M4 and variants fire 5.56x45mm NATO ammunition and are gas-operated, air-cooled,
magazine-fed, selective fire firearms with a 4-position telescoping stock. Original M4 models
had a flat-ended telescoping stock, but newer models are now equipped with a redesigned
telescoping stock that is slightly larger with curvature at the end.[2] The M4 is similar to much
earlier compact M16 versions, such as the 1960s-era XM177 family. Some of those visual
designs are obvious in both weapons, however most of the similarities are not very noticeable.

As with many carbines, the M4 is handy and more convenient to carry than a full-length rifle.
The price is slightly inferior ballistic performance compared to the full-size M16, with its nearly
6" longer barrel. This becomes most apparent at ranges of 300 yards and beyond.
Statistically, however, most small-arms engagements occur within 100 yards. This means that
the M4 is very much an adequate weapon for the majority of troops. The marginal sacrifice in
terminal ballistics and range, in exchange for greatly improved handling characteristics, is
usually thought to be a worthwhile compromise.

While the M4's maneuverability makes it a candidate for non-infantry troops (vehicle crews,
clerks and staff officers), it also makes it ideal for close quarters combat (CQC), and airborne
and special operations, which is the most popular usage for the weapon. It has been adopted
by United States Special Operations Command (USSOCOM) and is the preferred weapon of
the U.S. Army Special Forces and Navy SEALs. M4's have also been fielded by the Australian
Special Air Service Regiment. Malaysia purchased M4 Carbine service rifles to replace the
Steyr AUG service rifles in its armed forces in 2006 and will be manufactured in Malaysia
under license by SME Ordnance Sdn Bhd.[3] It is expected that the M4 will be adopted by law
enforcement agencies in Malaysia, such as the Royal Malaysian Police and the Malaysian
Maritime Enforcement Agency.[4]

The M4 was developed and produced for the United States government by Colt Firearms,
which has an exclusive contract to produce the M4 family of weapons through 2009; however,
a number of other manufacturers offer M4-like firearms. The M4A1, along with the M16A4, has
mostly replaced the M16A2; the U.S. Air Force, for example, plans to transition completely to
the M4 Carbine.

The United States Marine Corps has ordered its officers (up to the rank of lieutenant colonel)
and (senior) Staff Non-commissioned officers to carry the M4A1 carbine instead of the M9
handgun. This is in keeping with the Marine Corps motto, "Every Marine a rifleman." United
States Navy corpsmen will also be issued M4A1s instead of the M9.