The plain fact of the manner that everyone here has failed to mention is that all current 4K monitors do NOT support refresh rates higher than 60Hz @ 4K resolutions. If you step down the resolution to 2560 x 1440 (or other variant resolution lower than 4K) then you can game at higher refresh rates. Also, with H-IPS monitors @ 4K, the response times are HUGE. Usually a minimum of 12ms and usually more, versus only 6-8ms @ 2560 x 1440. Also, there is currently not one single 4K monitor that is a true single tile monitor. Even the coveted Asus ASUS PQ321Q uses a tiled display. IMHO, it is too early to adopt any 4K monitor for gaming until a true single tile 4K monitor is released with new IPS technology that does not exhibit the response time delays associated with current IPS technology and comes equipped with both HDMI 1.5 and DisplayPort 1.3 for the higher bandwidth requirements of 4K. DisplayPort 1.3 allows for the support of dynamic refresh rates as well as Nvidia's G-Sync technlogy. If you are able to live with 6-bit color, then I would highly recommend you get the Asus ROG SWIFT PG278Q ($799) 2560 x 1440 120Hz 1ms G-Sync TN panel monitor when it is released later this year:
http://www.asus.com/us/News/xXtX0FNhXQWPrry7
Or if you want H-IPS, then the Overlord Tempest X270OC ($519):
http://overlordcomputer.com/collections/27-monitors/products/tempest-x270oc-glossy