A 39" 4K has a 113 ppi (http://pixensity.com/list/seiki-digital-se39uy04-430/)

A 13" Macbook retina has a 227 ppi.

So even a 4K 39" monitor still has a far lower ppi than a retina display. And I absolutely can tell a retina display from a normal display. That means there's a lot of room for resolution to go up to and beyond 4K before it becomes indistinguishable to human eyes.

And I stand right by the monitor when I use my computer. Right now my monitor is about 12" away from my face.