I own an M1, shot an M3, and only gunshop fondled an M4.
I can't say which is 'better', and I don't know if you know the differences, but here are the differences.
M1 is your basic semi-auto Benelli. The reason I went M1 is SureFire has a weaponlight for it; they don't for the M3 or M4.
M3 has a switcheroo that converts it to pump for light loads, usually less-lethal stuff like teargas, beanbags, etc.
The M4 is gas operated and has an optic rail. It's gas operated, because I think it's designed to throw lots of doodads on there like optics--if you load the M1 too heavy it starts to get picky about what you feed it.
I never shot an M4, and the only ones in the US are pinned fake telestocks and crimped magtubes so you have the weight but not the capacity and the silly stock.