RE: broadheads hitting 4 inches low.
I would move the rest up ever so slightly and see what happens. You could also try moving the nock down just a little bit. Do one or the other. It sounds like you are almost there. Just remember that you have wings on the front of your arrow and vanes on the back. They will magnify any tuning problem. That's where I'd start. If it doesn't solve your problem, move your sights to address the issue and call it good.