The difference:
http://www.m4carbine.net/showthread.php?t=193
Generally if a upper receiver has the M4 cuts the advertising will definitely say so. If no mention is made then 99% chance they don't have them.
Original = carry handle, no forward assist
A1 = Vietnam era style with carry handle, and windage-only adjustment on rear sight
A2 = similar to A1 ut rear sight also adjusts for elevation
A3 = same as A1/A2 ut a picatinny rail instead of carry handle
M4 = same as A3 except with the cuts for the feed ramps that match the M4 barrel extension
I am not sure what some manufacturers mean by an "A4" receiver. I have NOT seen such a reference WITH a note that it has the M4 ramps, so I assume it is the same as other manufacturers A3.
Usually the normal cut ramps in the barrel is referred to as a "rifle barrel" and the barrels with the extended cuts that match the M4 receiver are called "M4 barrels". Some barrels as long as 20" have the M4 ramps. Many short barrels do NOT have the M4 ramps.
"M4" barrel or receiver ONLY refers to those with the extended feed ramps. Any other designation most likely means it does not have the extended feed ramps.
