wich is better for a m4 with a metal body,a metal hop up or a plastic hop up.someone pleease give me some advice,thanx

IMO, it dosen't matter. I'm not a armalite fanboy though. The major concern is that you can apply ample backspin to your bbs, even with abnormally heavy bbs for your velocity. I'd expect to loft .25g BBs at 350FPS and .3g at 400FPS. Unless you hopup unitl is broke or you can;t do anything to prevent it from backing off then replace it. But if you need more "hop" aka, backspin, then replace your hopup bucking.

A lot of people like SystemA hopup units they're are others out there but I here those are good and SystemA is one of the oldest and most trusted brands around.

It doesnt matter what they are made of but keep in mind that metal body's have a different style of hopup usually over plastic bodied M4s. Airsoft Atlanta carries a guarder hopup unit that is around $27 or so that is plastic and is far better than the CA unit. I also have seen a systema metal hopup unit for an M4 and was not impressed myself. For the $27 the guarder seems to be the best value that i have run into so far.