This is not a replacement for the M-4, it is replacing the M249 Light Machine Gun, itself a 5.56 weapon. This is the equivalent of the Army's SAW
The Commandant intends to equip every Marine in an infantry squad with an M27, not just the squad gunner. The decision on who outside of the infantry squad gets one is still being discussed.
https://www.military.com/kitup/2018/01/05/m27s-and-head-toe-gear-overhaul-way-marine-grunts.html