The reason the M4 is less accurate is because of the barrel twist. It only has a 1:7 twist, which is not enough to stabilize the round before it leaves the barrel. The 1:7 twist is fine if you have a longer barrel, but the 14.5" barrel of the M4 is not enough.
A guy I know has an M4 that has a barrel twist of 1:9 and he says it is more accurate than the standard M16.
Of course that all depends on the type of ammo being used. Since standardization is king in the US military, the rounds they use are 62gr M855, which is fine for the M16, but less accurate for the M4 at range.
Some gucci spec ops guys are getting this kit, and I hear they all love that ACOG scope.
The company Barrett is actually making an "improved" M4 that is going to be of a higher caliber. Supposedly it will behave like the 7.62 rounds, but smaller/lighter. It's supposed to be 115 or 120gr I think.