(in addition to select fire capability) M4's also have adjustable stocks, different feed ramps, and a carbine length gas system. The barrel also has a cutout for M203 grenade launchers.
People often refer to civilian legal m4 looking rifles as M4geries. They typically have M4 feed ramps, detachable carry handles, carbine length gas systems, 14.5" barrels with pinned on flash suppressors to make the barrel the legal 16", m203 cutouts, and collapsible stocks. In short, the Colt 6920.