You would think the M16A4 would be more advanced than the M16A3 due to it being the newer version, and in real life this is likely the case. In Battlefield 3, the 2 weapons are almost identical except for their fire modes (see below). Although the Wikia page for the M16 mentions that the M16A3 has a slightly faster empty reload time , the statistics listed under each variant are exactly the same – 1.80 loaded and 2.37 empty.
M16A3 Fire Modes: Semi Automatic/Automatic
M16A4 Fire Modes: Semi Automatic/3 Round Burst
After using the M16A3 and getting used to pelting enemies with automatic fire it took some getting used to 3 round burst mode.