I have a dual monitor setup, a 28inch 4K 60hz and a 27 inch 1440p 165hz.
From my sitting position, I don’t notice too much difference in terms of image quality between these two especially when I am gaming, I will watch most of my videos on the 4K one. But when it comes to gaming, the high refresh rate really makes a night and day difference. My rig is i7 8700k, and 1080ti ftw3, I barely hit 60 FPS in 4K for most of titles. If you are going for 4K 120hz, you will need a much beefier setup. If I were you, I will just get 1440p high refresh rate right now. It is quiet a experience. And do get a IPS and gsync, it is essential.