I need a recommendation for a video card for a MacPro5,1 that supports Metal for the new Mojave OSX and also supports 4k. I don't need the top of the line card but preferably the cheapest one.

Depends on what you have, but probably, yes. Nothing Apple sold in the Mac Pro 5,1 will support 4k or 5k video. What would probably be the minimum video card I would need to run 4k and probably a ...