In singleplayer, the M4A1 (with a Tasco Red Dot Sight, AN/PEQ-2A, and foregrip) is a frequently dropped weapon among theMarines, and an infrequently dropped weapon among the SAS troops. It is the starting weapon in Charlie Don't Surf and Game Over, and one of the optional weapons at the beginning of Hunted. The "M4A1 Grenadier", which uses an EOTech Holographic Weapon Sight, AN/PEQ-2A, and M203, is the starting weapon in The Bog, War Pig and Shock and Awe. The "M4A1 SOPMOD" uses the red dot sight, M203 grenade launcher, AN/PEQ-2A infrared laser, and silencer, and is the starting weapon in the majority of the British missions. Captain Price uses it in all missions. Surprisingly, in the SAS missions, the M4A1 SOPMOD seems to have all attachments available, a red dot sight, silencer, grip (though the M4A1 Carbine has a grip, this does not serve to decrease recoil and does not count as an attachment. Note that the grip does not show from the third-person perspective in-game, as when dropped or held by another character, in both single-player and multiplayer, and an M203.

In multiplayer, the M4A1 Carbine has an attached foregrip. It can be modified with the Red Dot Sight, ACOG Scope, M203, orsilencer. The M4A1 Carbine is fully automatic with moderate damage and very low recoil, but it has very high idle sway (4 degrees) and idles very fast, making it more difficult to aim with at long range. This was done for balancing purposes because the M4A1 was incredibly overpowered in the beta, in which it had no idle sway. The M4A1 also shares ammo with the M16A4 and the G36C. The M4A1 Carbine is one of the most commonly used weapons in online multiplayer on all platforms.

As stated above, the M4A1 is not particularly effective at long range due to its sway on consoles. In the PC version, however, idle sway is easily compensated for with the mouse, making it very effective at all ranges. The same goes for the Wii version; if you have a steady hand you can control the sway with the Wii remote, making the weapon devastating. The M4 is ideal in close quarters combat due to its high rate of fire. Steady Aim also makes the M4 a fairly effective hip-fire weapon. Double Tap can be quite effective due to low recoil and low damage, but ammo use increases dramatically. Another VERY good perk is stopping power for the rifle, which makes it deadly at all ranges.  

The weapon's low damage output can be compensated by its high rate of fire, and the very low recoil allows for accurate full automatic fire at short and medium ranges. Because of this, Bandolier can be useful. For these reasons, a silencer or a red dot sight is a good choice.

 The M4A1 returns in Modern Warfare 2, and has retained most qualities and appearances of the Call of Duty 4 version. Although it seems to have more of a tactical appearance. The M4 in this game is actually very similar to the Bushmaster Modular M4 Carbine, except that the game version has a shorter barrel, indicating that it is a version built for close combat. The AR-15 carry handle which was present in Call of Duty 4 has been replaced with foldable PRI front and ARMS #40L rear sights and the Knight's Armament Rail Interface System (RIS) has been replaced with an ARMS Selective Integrated Rail (SIR) System. It now sports tan camouflage, as opposed to the previous gunmetal color. The M4A1 retains the foregrip, although it is absent in the create-a-class profile image. The M4 comes in many variants in the single player campaign, such as the M4A1 SOPMOD and the M4 Grenadier w/ Holographic, usually featuring more than one add-on.

Trivia

• On the lower receiver of the M4A1, it says "BLUE4"(a reference to BluFor, the opposite of OpFor, which is applied to all NATO forces) which is the same for the M16A4. It also says, "Property of U.S. Government, Caliber 5.55mm, M4A1 Carbine" over the magazine housing. However, the M4A1 Carbine fires a 5.56mm, not a 5.55mm, and it is also used by the SAS in game. It saying "5.55" is most likely a typo or part of a way to avoid copyright infringement.
• If you set the sensitivity on console versions to 10 (max) then spin in circles clockwise (to the right) the right side of the rifle can be slightly visible, showing a much less detailed texture. This was done because that side of the rifle is not normally visible and not detailing it saves memory.
• There is a special version of the M4A1 called the M4A1 Suppressed. NPCs use it and it can only be acquired by using cheats. (It's also in the Museum level.)
• In Modern Warfare 2, the rifle sounds like the Call of Duty 4 M4A1 in the singleplayer campaign, but in multiplayer, the rifle sounds quite different, with a lot more punch in the sound.
• In the beginning of the Modern Warfare 2 level "The Only Easy Day...Was Yesterday" The M4A1 is titled as M4A1 SOPMOD but later in the level it changes to a M4A1 Grenadier w/ Red Dot Sight and loses its Suppressor attachment after you detonate the corpses. The SCAR-H also loses its suppressor at this point. However, if you switch out the M4A1 for another rifle and switch it back after ambush it will still have the suppressor attachment; the quirk applies for the SCAR-H.
• When attached with the FMJ, the grip on the rifle disappears when it is dropped, reloaded or in 3rd Person but a removable olive green grip is seen when held. It is easy to see with insane look sensitivity by looking left and right and the player seems to hold the grip.
• In Modern Warfare 2 multiplayer, the name is M4A1. However, with an ACOG scope, the name won't be "M4A1 + ACOG scope" but "M4 + ACOG scope".
• In Call of Duty 4, the third person view of the M4A1 lacks the grip, and NPCs and players hold the weapon like there was not a grip. In Modern Warfare 2, the grip is visible in third person but is still not used.
• In MW2 the grip disappears when a Thermal Scope is attached,but the character model still holds the gun as if there is a grip.
• In the campaign in MW2, when using the M4A1 Grenadier w/ Red Dot Sight variant, aiming down the sights provides further zoom than a normal red dot sight.