The barrel extension (the usually black part on the back of the barrel) on an M4 barrel are cut a little deeper than on the standard rifle barrel.
The upper receiver on an M4 has two small cuts under the barrel socket that provide additional clearance for rounds in the magazine to feed into the barrel extension.
Regardless of what you hear to the contrary, it helps feed hollow point (match) ammo better and gives older, worn magazines a longer life.
I have found the ramps to be helpful in both M4s and AKs, when hollow point (match) ammo is used.
I would opt for the anodized M4 ramps, if you have a choice, on the upper receiver, provided you use a barrel with M4 ramps.
If you use a barrel with standard rifle feed ramps (no ramps), I recommend you do not use an M4 cut upper.