The M4 is an American carbine which has been battle tested nearly as much as the AK-47. The M4's overall length and firing speed is the main downside as near corners it must be lowered and slow time between shots. The M4 is extremely effective over range and engaging armored targets.
The M4's stats are among the best for full accuracy and stopping power. These stats make many of the rifles available for the Assaulter class virtually pointless within the scope of the game. Many of the other rifles just have overall worse stats such as the MK 17. Other rifles have a single point gained in maneuverability while losing a point in stopping power or accuracy such as the HK416D10.