Good question ! I don't think there's that much of a difference. I think it's the noise off it that a few of my friends think it's much faster,if any. I know what the zed can do to certain fixed points & I don't think the M4 is that much quicker.
The handling & brakes Are the big improvement over the zed. It was a itch I had to scratch,but don't think I'll be ordering a M4 at the moment. Saying that if I go into the dealer today & he gives me a fantastic deal
lol

