I'm wondering to those who have a 4K monitor, when do you find it useful?
4K monitors of sizes 27" and less are typically not about screen space, but about clarity and detail.
For example, I plan to purchase a 4K monitor to use it with 200% scale enabled on OS level. So I will have as much screen space as Full HD provides at 100% scale, but with much more clarity and detail — for text and images in particular.
Also, flickering caused by crystal inversion should be less noticeable thanks to some intercompensation of visually partially blended adjacent pixels (that are much smaller than on non-UHD monitors) that are in antiphase.
And finally invisible interpixel grid which is somewhat annoying on non-UHD monitors.