A rifle, of the general configuration of the Colt M4(tm), that is not actually made by Colt.
An M4 is generally considered to be of the M16 family and having the following features: A short gas system, a barrel weighted in front of the front sight post with a cut out to facilitate the mounting of an M203 grenade launcher, an adjustable stock and feed ramps that extend from the barrel extension into the upper receiver.
"That kool-aid drinking homo said my rifle is not an M4 because my feed ramps were not carved by Samuel Colt himself"
A civilian AR-15 rifle that has been configured as the military M-4 carbine with 14.5" barrel, flash supressor to bring barrel length to 16", and collapsable stock.
This M4gery tucks better than my AK.