I would strongly suggest looking at the Monoprice house brand monitors.
Since you seem to be looking around the $400 price point, you could get into their 32" units, which are the best size to start moving up to 4K. Totally agree with others who say 27" is too small, at least on Windows.
In your position, I would probably grab this 32-inch monitor
, which hits all the major points: IPS, LED, 2x HDMI, 2x DisplayPort, 4K, HDR, and FreeSync. Monoprice also has a zero dead pixels policy, which I consider very important for my primary display.
I have an older model (no HDR), and it's top notch.