One of the bigger WE M4 suppliers, www.airsoftbuddy.com
, has those main o-rings (part #122), available as a replacement, 5 for five bucks, plus shipping
If you haven't already, get onto Arnies Airsoft and check out their WE m4 review thread. Guys have run into some of these problems before, and come up with easy fixes already. The shredded o-ring can be prevented by using fine snadpaper to de-burr the part that o-ring slides into. I'll try and dig up some more precise info...
WEs' website has a Exploded Parts Diagram
for the M4