The M4 carbine (officially Carbine, Caliber 5.56 mm, M4) is an assault rifle developed in the United States during the 1980s. It is a shortened version of the M16A2 assault rifle. The M4 is extensively used by the US military, with decisions to largely replace the M16 rifle in US Army (starting 2010) and US Marine Corps (starting 2016) combat units as the primary infantry weapon and service rifle. The M4 has been adopted by over 60 countries worldwide, and has been described as "one of the defining firearms of the 21st century".

Since its adoption in 1994, the M4 has undergone over 90 modifications to improve the weapon's adaptability, ergonomics and modularity, including: the M4A1, which possesses a thicker barrel and a replacement of the burst-fire control group with a fully automatic one; the SOPMOD, an accessory kit containing optical attachments; and the underbarrel weapons such as M203 and M320 grenade launchers to the Masterkey and M26-MASS shotguns.

In April 2022, the U.S. Army selected the M7 rifle, a variant of the SIG MCX Spear, as the winner of the Next Generation Squad Weapon Program to replace the M16/M4.

Following the military adoption of the Armalite AR-15 as the M16 rifle, carbine variants were also adopted for CQC operations, the first of which was the CAR-15 family of weapons, which was used in the Vietnam War. However, these rifles had design issues, as the barrel length was halved to 10 inches (25 cm), which upset the ballistics, reducing its range and accuracy and leading to considerable muzzle flash and blast, meaning that a large flash suppressor had to be fitted.

In 1982, the U.S. Government requested Colt to make a carbine version of the M16A2. At the time, the Colt M16A2 was the Colt 645, also known as the M16A1E1. Later that year, the U.S. Army Armament Munitions Chemical Command helped Colt develop a new variant of the XM177E2, and the U.S. Army redesignated the XM177E2 to the XM4 Carbine, giving the name as the successor to the M3 carbine. The carbine used the same upper and lower receiver as the M16A1, and fires the M855 cartridge along with the older M193 cartridges. In 1983, the 9th Infantry Division requested a Quick Reaction Program (QRP) for a 5.56mm carbine to replace the M1 carbine and M3 submachine gun in service. The XM4 was tested by the Army's Armament Research and Development Center (ARDC) in June 1983. Later, the gun was updated with improved furniture, and a barrel with rifling of 1 turn in 7 inches (180 mm). The ARDC recommended additional commonality with the M16A2 rifle, as well as lengthening the barrel to 14.5 inches (370 mm). In January 1984, the U.S. Army revised the QRP, and a month later, it formally approved development of the new carbine.

In June 1985, the Picatinny Arsenal was given a contract to produce 40 prototypes of the XM4. Initially a joint program between the Army and Marines, in 1986 the Army withdrew their funding. The XM4 was finished in 1987, and the Marines adopted 892 for that fiscal year, with the designation "carbine, 5.56mm, M4". Owing to experience from the 1991 Gulf War, the Army gave Colt its first production contracts for M4 carbines in May and July 1993, and M4A1 carbines for United States Special Operations Command (SOCOM) operators in February 1994.

Interest in the M4 carbine was accelerated after the Battle of Mogadishu (1993), in which Rangers complained that their M16 rifles were "unwieldy", whereas members of Delta Force in the same battle, equipped with the CAR-15, had no such complaints. The M4 carbine first saw action in the hands of U.S. troops deployed to Kosovo in 1999 in support of the NATO-led Kosovo Force. It would subsequently be used heavily by U.S. forces during the war on terror, including in Operation Enduring Freedom and the Iraq War. In the Army, the M4 had largely replaced M16A2s as the primary weapon of forward deployed personnel by 2005. The M4 carbine also replaced most submachine guns and selected handguns in U.S. military service, as it fires more effective rifle ammunition that offers superior stopping power and is better able to penetrate modern body armor.

In 2007, the USMC ordered its officers (up to the rank of lieutenant colonel) and staff non-commissioned officers to carry the M4 carbine instead of the M9 handgun. This is in keeping with the Marine Corps doctrine, "Every Marine a rifleman." The Marine Corps, however, chose the full-sized M16A4 over the M4 as its standard infantry rifle. United States Navy corpsmen E5 and below are also issued M4s instead of the M9. While ordinary riflemen in the Marine Corps were armed with M16A4s, M4s were fielded by troops in positions where a full-length rifle would be too bulky, including vehicle operators, fireteam and squad leaders. As of 2013, the U.S. Marine Corps had 80,000 M4 carbines in their inventory.