You need an HDMI cable that supports it for one. Only version 1.4 and up do. Version 1.4 only supports 4K at 30 Hz. It takes version 2.0 to support 4K at 60 Hz. You also need a pretty hefty GPU to support 4K 60. DisplayPort also supports 4K, but very few TVs have it.
If you need to get a HDMI 2.0 cable, meantime you can enable DSR with an Nvidia GPU or VSR with an AMD one. The more options you check, the more res choices you'll have in game. I use DSR a fair amount on my Panasonic 1080p HDTV on my 1080 SC. It's very close to the look and resource use of the res it simulates. I'm perfectly fine using it until UHD broadcasts and UHD tuner eqipped TVs become available.
DSR...