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.
The original poster wants a monitor. the child comment talked about a tv. Correct me if I am wrong but HDMI (before 2) cannot deliver 4k at 240Hz, right? Has that changed with the current iteration?