Peteruagusts, perhaps "M4" is misleading. The OP is after the flat-top AR look. If you go to the URL the OP provided you'll see what he has in mind. It's like this
image: expertoutfitters
but includes a halo sight in addition to the A2-type clamp on. It's not my cup of tea; but, if it was, I'd just get the gun in question without trying to modify a R-870.
Different strokes for different folks.