M16A4 > M16A3
M16A4 has lower ADS spread.
M16A4 has less recoil.
M16A4 actually exists, whereas M16A3 should be modeled after the M16A2.

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(The M16A3 is a fully automatic variant of the M16A2 adopted in small numbers around the time of the introduction of the M16A2, primarily by the U.S. Navy for use by SEAL, Seabee, and Security units. It features the M16A1 trigger group providing "safe", "semi-automatic", and "fully automatic" modes.)

The M16A4 is the fourth generation of the M16 series. It is equipped with removable carrying handle, and full length quad Picatinny rail for mounting optics and other ancillary devices. The FN M16A4, using safe/semi/burst selective fire, is now the standard issue for all U.S. Marine Corps and a few select U.S. Army units, and is the current issue to Marine Corps' recruits in both MCRD San Diego and MCRD Parris Island. Military issue rifles are also equipped with a Knight's Armament Company M5 RAS hand guard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached, coining the designation M16A4 MWS (or Modular Weapon System) in U.S. Army field manuals. The Marine Corps is considering a Product Improvement Program (PIP) for their M16A4 MWS rifles. Potential features include a modular and adjustable stock, ambidextrous fire selector, heavier barrel, improved trigger, free-floating rail system, and an adjustable gas block and gas regulator to give all rifles a suppression capability.