are you saying you can't get enough frames on the 5K monitor? and that's your only issue? i've heard the input lag is terrible on those 5K displays..
also, there are no 4K displays that can do 120Hz+ as of now. There is one in development that has been posted already.
If input lag is not an issue for you, as well as the monitor being 60Hz vs 120Hz, i would highly suggest checking out something in the 40" 4K size.
This is a new AOC monitor that looks pretty decent. 40" 4K curved 60Hz. Since you don't have to worry about windows scaling, productivity on this thing is AMAZING.
here's OC3d's article on it