Re contouring an M4 probably isn't worth the risk of having a friend run it through a lathe especiailly since there wouldn't be that dramatic a weight reduction. It's harder than it seems if he hasn't got experience. There is no guarantee that whatever accuracy potential the barrel has now, will be retained.
Even having one of the highly regarded folks on this board do it would have some risk, and would run prob. $100 plus.
If you're desperate, consider selling your barrel and buying an lightweight, say from CMMG. If you like to gamble, go ahead and "turn" your barrel; good luck!