The major difference between these models is that the M4 has a "S-1-3" (safe/semi-automatic/3-round burst) trigger category while the M4A1 has a "S-1-F" (safe/semi-automatic/fully automatic) trigger group.
This kit was designed to be configurable (modular) for various missions, and the kit is currently in service with special operations units (though http://www.m4forsale.com multitudinous brigade have changed the Trijicon reflex appearance for EOTech holographic sights).