Is 4K monitor a good replacement for Retina display?
A 13" Retina MacBook Pro has a screen resolution of 2,560 × 1,600, while the 15" has a 2,880 × 1,800 resolution. 4K is defined as a 4096 x 2160 resolution.

So depending on how big the monitor is, a 4K monitor should actually look better from the same viewing distance.

As for my personal recommendation, anything but a Samsung.