Okay, I'm not an M4 guy, but I'll take a shot at it.
A true M4 barrel is 14.5" long, has a bayonet lug, a flash hider and is contoured to accept a grenade launcher. Available to military, police and licensed individuals only, because the barrel is less than 16" overall.
The M4 barrel available to us common folk is 16" overall length including the muzzle break (flash hider for a pre ban lower reciever). No bayonet lug (pre ban can have one). Purely cosmetic.
But looks good, even if it is fake.
By the way, rifles in this configuration are commonly known around here as M4geries.