The M16A4 did not feature any radical changes as far as the operating system compared to the M16A2, but the two changes that were made have helped it to become a much better weapon system.
Changes include a flat top upper for mounting of optics or a detachable carry handle and KAC M5 RAS rail system. Like the M16A2, Selector switch is safe, semi, and burst (3 round).
These rifles are being fielded mostly by the USMC and the vast majority of them are currently being made by FN (Fabrique Nationale) for the US Military. However, the full size M16 rifles like these are starting to lose favor in the military, as many units are getting more and more M4 Carbines.
More on the evolution of the original AR15/M16 to come.