Not sure but there are some 4 rail fore end options for M4s. Can be kind of pricey. I bought a KZ 4 rail fore end for 200 bucks, it is all steel and mounts solid as a rock.
Duracoat is a finish specifically developed for use on guns. There are about 100 colors I think. Have to web search for a dealer that offers the surface near you, but I have seen some pretty cool looking designs and it is not to exspensive from what I have seen.