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I used a Aimpoint during hunting season. I had read on the Benelli forum that the rail was not a picatinny? I used a LaRue mount with no problems. Check out his FastFire IronDot.
If you don't mind spending the money, you really can't go wrong with an EOTech. Very fast sight picture in all light conditions. I've used both an EOTech and a docter and I prefer the eotech. But I'd say try both if you can and see which you like better. Order one of each and return the one you don't like!
The Docter optic is an excellent choice for the M4 and the quality is great.
Just do not drop it too often since they are a bit more fragile than the Eotechs and the Aimpoints.