Muzzys are great heads, and I've thought about MX-4s myself... although in my experience they are among the harder broadheads to get consistantly great flight with. A lot of that is due to that trocar tip that sits out front. If your arrows are not perfectly square and spined well, and if your bow is not tuned correctly, then you'll get some wobble.

Are your arrows grouping consistantly, but just not in the same place, or are they just all over the map?