Unless the lowers were milspec lowers intended for use with a three-position selector lever, they're not true M4 lowers. A true "M4" is the military carbine version that's capable of full-auto or burst. That said, "M4" is a marketing term thrown around by many manufacturers of AR-15s.
The terms marked on lower receivers normally mean absolutely nothing. For example, some standard Colt 6920s come with lower receivers stamped "Law Enforcement Carbine", some are marked "Sporter", some are marked "M4 Carbine Sporter", and some are marked "M4 Carbine". What's the difference between those rifles? Absolutely nothing at all, other than the markings on the lower.
0331: "Accuracy by volume."