The M4 S-System's rail system attaches to the body in a very specific way. You need to get a specific metal body... I think there are two or three different ones on the market right now. If it were my decision, I'd get a CA or a G&P, and get a Samurai SIR. (
http://www.wgcshop.com/pcart/shopper...tm=SAM-ACC-SIR) A teammate has one on an ICS M4, it looks spectacular and it's tough as nails, tougher than the TM one in my opinion.