My friend got a Samsung UN40KU6290 4K 40" TV as a monitor for around $250 during the holiday sales, and I think it's an absolute steal even at the regular price of $300. It does chroma 4:4:4, has best of class input lag, and the picture quality is decent too. But most importantly, you get the resolution of 4 1080P monitors for the price of 2, and the size is large enough to comfortably make use of all the desktop real estate without having to mess with scaling.

It really baffles me how they can afford to sell it at that price.