A 27 inch 4K monitor becomes retina at about 21 inches away. So in that sense you can say it qualifies as a Retina display. I myself have a 27 inch 4K monitor from LG and I can't even differentiate the pixels when I put my face up to it.
But also it's almost a full $1000 cheaper.
I've got a Dell 27" 4K monitor next to an LG 5K. I've scaled it so the windows appear the same size and you can definitely see that the images on the Dell are ever so slightly fuzzier than the LG.