4K alone is not enough to define spatial resolution. You also need take into account physical dimensions. DPI is a better way to describe spatial resolution. Anything better than 200 DPI is good, better than 300 is awesome.

Unfortunately, there are no 4K displays with 200+ DPI on the market. If you want high DPI you either need pick glossy 5k@27" or go to 6k/8k.

People with 240Hz or higher monitors. 4K is enough for spatial resolution then it is better to increase temporal resolution. 4K at 240Hz or higher looks like a window.