If Benelli offered the M4 with military/LE configuration to the public in the USA, I could see buying being more likely. At $2,000 "street pricing" plus all the money to restore it to its original configuration (or current LE configuration) it is simply not price competitive with the Beretta 1301.
The ~1 pound weight difference is not significant to most people but, that is a tangible difference just like the added mag capacity from the factory configuration. Add the stuff missing on the M4 and you are much further apart than the $400 initial price difference.
Add to all that, the fact we civilians are not flying all over the Mideast or to the Arctic Circle and don't deploy on multi-day missions in hostile territory. My 1301 might need some cleaning before >20K rounds of buckshot but, how many years will it take me to shoot that much? What is the likelihood I will shoot >100 rounds without a full cleaning?
Finally, the 1301 series is built for a wide variety of users and different types of shooting which to me makes it more versatile overall. I voted with my wallet but, could have easily gone with the 300 Patrol shotgun too without giving up much, leaving a lot more money for shotgun shells and training. A bit like the poor college student in the old Subaru on the Forestry Service road with a tent ... A nice Lexus Overlander would be sweet but, would they have any gas money left to actually travel?