I by dumb luck found a way to get 4K@60Hz for my RCA 55" HDMI 2.0 input TV:
http://www.microcenter.com/product/453009/LED55G65RQ_55_4K_Ultra_HD_LED_TV Any of my late model Nvidia or AMD graphics cards with HDMI output would do 4K@30Hz, but not 60Hz. I tried a Quadro K420 (entry level workstation card) and the Displayport to HDMI adapters did not work even at 4K@30Hz with this card. However, I plugged the RCA TV into the DL-DVI port on the K420 with a cheap passive DVI to HDMI adapter block, and presto, 4K@60Hz on the RCA TV!
I also confirmed this works with a Quadro K620.
Now if only I can get the same thing from other video cards with HDMI output, or Display Port as the OP asked.