The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16 rifle, all based on the original AR-15 rifle designed by Eugene Stoner and made by ArmaLite. The M4 is a shorter and lighter variant of the M16A2 assault rifle, with 80% parts commonality. It is a gas-operated, magazine-fed, selective fire, shoulder-fired weapon with a telescoping stock. A shortened variant of the M16A2 rifle, the M4 has a 14.5 in (370 mm) barrel, allowing its user to better operate in close quarters combat. Like the rest of the M16 family, it fires the .223 caliber, or 5.56mm NATO round. The M4 has selective fire options including semi-automatic and three-round burst (like the M16A2 and M16A4), while the M4A1 has the capability to fire fully automatic instead of three-round burst (like the M16A1 and M16A3). The carbine is also capable of mounting an M203 grenade launcher (the M203A1 with a 9-inch barrel as opposed to the standard 12-inch barrel of the M203 used on the M16 series) as well as its successor, the M320 grenade launcher.

Season 2

An M4A1 Carbine is used by Tuco Salamanca in "Grilled", when he takes Walter White and Jesse Pinkman hostage. Walt later takes this weapon from him when Tuco is bashed over the head with a rock by Jesse. However, Tuco gets it out of the car when DEA agent Hank, who is also Walt's brother-in-law, arrives at Tuco's hideout. While making his report to his superiors, Hank misidentifies the weapon as an M16, the M4's heavier, longer-barreled counterpart, which is incapable of fully automatic fire. DEA agents also use M4A1s when they arrest Jesse in "Bit by a Dead Bee".

Season 4

The M4A1 appears again in Cornered in the hands of one of Gustavo Fring's gunmen riding in the back of a refrigerated truck

Season 5

The M4A1 appears in the final scene of "To'hajiilee" and the beginning of "Ozymandias" in the hands of two White Supremacists who ambush Hank and Gomez.