Topic, if you made that yourself then all you need are a few brilliant ideas to create the perfect M4 (or any gun, really). You did a magnificent job, and if you could use some help with ideas, look no further - I am the master of spontaneity and thus my creativity will be of much use.
As for criticism..
Well, the colors just don't fit with the M4 - especially the purple. The colors also don't flow too well; they're just kinda.. there. It's a bit messy with that regard, and it simply doesn't appeal aesthetically to me.