I wouldn't bother with a 4K monitor - you might be able to get nine tables on it, but the buttons and text will be small. Better off with two 1080ps that can fit eight tables at a more friendly size.
It's really difficult to play across two screens and keep track of the action. I have one for the lobbies of the clients and skype / web / range charts, the other screen is purely for the tables. The absolute minimum requirement is a 27" 2560 x 1440 to be able to tile 12 on there clear enough to see everything easily.