If you cannot see your desired resolution on the list e.g. 4K, simply hold Option key (on Mac keyboard) or ALT key (on Windows keyboard), then click Scaled radio button.

A list of all supported resolutions will be shown. Try not to use unsupported resolutions with (low resoluton) description or you might end up with black screen.

If you still don't see 4K resolution on the list, make sure you are using DisplayPort to DisplayPort cable (or miniDisplayPort equivalent).

If you have HDMI 2.0 capable display you can use it too (on Polaris/Vega cards you need to use an active DisplayPort to HDMI 2.0 adapter to get full resolution with 60 Hz refresh rate). Some GeForce/Quadro cards only support 1080p via HDMI port.