I have 2 large 4K TV's I'm using as monitors. I'm using XFX R9 290 & 290X GPU's to drive them. I was using a MSI HD 7950 with one of them with great results until I upgraded. A used HD 7950 or 7970 can be found for under $130 with a bit of searching. It's been my experience the AMD-based GPU's do better at 4K than the Nvidia offerings.
i7 5930K @4.6ghz, Asus X99 Deluxe, 64GB G.Skill Ripjaws 4 Series 2400, XFX R9 290X, Samsung 850 Pro 256gb, 4 Seagate 7200 rpm 2tb, Seasonic X1250, Corsair H105, Asus Xonar Essence STX, LiteOn iHBS212, Plextor 890SA, Corsair 500R, Win 10 Enterprise 64.