dell monitor

I have got a new hackintosh, my first one, and a new monitor, a Dell 4k. On disc 1 I have el capitan and on a other one I have sierra, but they both don't recognize my monitor. Only some standard monitor. Can someone give me the solution please, it feels somewhat uneasy to have a state of the art monitor and meanwhile not having the pixels on my screen.
Thank you very much, Hjalmar