If you want to get a 4K reference monitor, then be prepared to pay big money. FSI is finallt about to release their first 4K reference monitor, and that goes for us$45.000
And if you look at the other brands, like Sony or TVlogic then its not anything less.
FSI best current monitors the DM240 (LCD) and DM250 (OLED) then they are both 1080 monitors, and i would argue that color accuracy and consistency is more important than resolution... or at least at the current prices it is :-D
They take an 2K input, and there is a firmware in beta that will make them take an 4K input, not that i would suggest you use that unless you have a backup reference monitor ;-)