That might be due to HDMI, if I remember correctly HDMI 2 supports 4k at 60hz, but HDMI 1 can only do 30hz. If you have display port on it, use that instead and you should be good.

I have two Samsung 4k monitors and they are unbearable to use with HDMI at 4k, I had to get a new video card to drive them both over display port.