According to the Marine Corps Times:
The recommendation to swap the venerated rifle that has served as the grunt’s primary implement of war since Vietnam now sits on the commandant’s desk, pending his final review and a decision. But, the swap appears imminent and if approved will relegate the M16 to a support role. It follows a similar shift already underway in the Army.
… The change would be welcomed by infantrymen who say the M16A4 was too long and unwieldy for close-quarters battle in Iraq or vehicle-borne operations in Afghanistan. They tout the M4 for its weight savings, improved mobility and collapsible butt stock, allowing the rifle to be tailored for smaller Marines or those wearing body armor.
“The carbine is a great weapon system for its time,” 1st Marine Division gunner, Chief Warrant Officer 5 Vince Kyzer told the Marine Corps Times. “… It will increase the war fighter’s lethality and mobility.”