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.

If you still don't see 4K resolution on the list make sure you are using DisplayPort to DisplayPort cable (or miniDisplayPort equivalent) as HDMI on newest cards doesn't support anything higher than 1080p resolution.