One of the configurations Colt sold their ARs is AR15A2 HBARs which means they have an A2 carry handle HBAR (heavy barrel) upper. M4 is a barrel profile, meaning that its thin (~0.60") under the handguards and has the M203 grenade launcher cut about 2" in front of the front sight. As far as a complete upper goes (upper receiver, barrel, handguards, gas block/front sight, gas tube, barrel nut, delta ring, handguard cap, bolt, bolt carrier and charging handle all assembled, ready to be put on a lower), M4 style flat-top uppers can be had from Bushmaster, Colt, DPMS, Olympic Arms, and a couple of others. Most complete factory uppers will cost close to $500, but they are factory assembled and will work. Bushies are usually pretty steep, but you can usually find a few others that are cheaper. CMMG has DPMS Pre Ban 16" M4A3 1 in 7" twist, chrome lined barrel uppers for $499, and they will throw in a carry handle for I think $75. Link: http://www.cmmginc.com/inventory_a/uppers_tbl.html
Hope this helps.
Ghost