You can get the M2 Tactical with ghost ring sights and it'll shoot slugs and buck shot just as well as the M4 will, and a pound and a half lighter.
With that said, I love my M4.
Both are great shotguns, the extra weight of the M4 takes a bit of the recoil out, but the M2 may be a bit better for long term hauling. And with the M2 you can take the rear ghost ring sight off, and change out the barrel to a longer barrel (Not that they're cheap or anything, but the M4 has 1 flavor of barrel)