i havent seen anything like that for the M4... ive seen a guy ( on the benelli forums online) with an adaptor to go onto the magazine.. its got 2 rails on either side of it to mount accessories.... i dont know much about it but looks to me like it would be a balls to take off the barrel as the rail would have to be takin off every time.
personally ill be getting the surefire handguard...its not that expensive ( B&T have a quad rail i can get but its €300 and has to be takin off to take off barrel, heavy and cuts the hands off ya.) and does seem to be the better (cheaper, lighter, quality make from a quality comany, value for money.) option out there ..
hope that helps...