Well, for 4K monitor you will need an appropriate video card and DisplayPort 1.2 @ 60Hz or latest HDMI that can handle 4K @ 60 Hz. Do not get anything less than 60Hz - you will kill your eyes over time with low refresh rate.
Resolution + bigger screen size → this is what I would go for. 60/120/240Hz refresh rate → there would be no difference making staff in UE. But screen size would make a big difference. Here is a screenshot of my desktop. See it yourself.
