I don't know that anyone has done this to an M4- but it is a re-packaged 760:

http://www.airgunhome.com/agforum/vi...light=pump+co2

One problem you will encounter- not enough meat on the valve head to drill and fit a connector in my view- do you have access to a MIG welder and some aluminum wire? (build up the head/cap of the valve and drill through) or a lathe to machine a custom cap (mind you, if you had these tools I guess you wouldn't need a new valve) ?