You cannot do 4K resolution on a computer monitor using the HDMI port. You have to use the DisplayPort for 4K monitors. HDMI does not have the bandwidth to support the 4K resolution.
If your video card does not have ta DisplayPort, then you will be looking at a video card upgrade, along with a cable.
Puma won't get 120Hz though, that's a TV feature, he'll only get 60Hz with a PC.