If I'm not mistaken Zeke makes metal bodies for the M4S that are availiable via WGC. A&A or ASCA shouldbe able to order the body for you, but by the same token I don't imagine there is some sort of drop-in kit for standard metal bodies.
My understanding of th M4S is that the upper dosen't actually have a rail on it, like your standard AR reciver, the SIR is actually the top rail, not clamped on like an ICS SIR.
http://www.wgcshop.com/pcart/shopper...Metal%20Bodies
Just a heads up, WGC is sold out all of all M4S metal bodies. Maybe UNC has them?
- Mike