Carver

First and foremost, the M4 (or any shotgun chambered for 3" shells) will not have been proof tested for a 3-1/2" shell. Essentially, you will be converting your M4 into a pipe bomb. At the very least the spent shell will jamb and not eject. I am sure, also, that Benelli would not pay for any repairs to a damaged gun or to a damaged user! Finally, why would you want to do that? Any advantage you would gain from the extra shot in a 3-1/2" shell would be nullified by the inherent danger.