I think you're reading the chart wrong. It says that if your screen size is ~30 inches, you will see the full difference of 4k if you are closer than 2.5 feet.
I assume that most people sit closer than 2.5 feet from their computer monitor, so unless you are playing on a 4k gameboy, I'd assume that any computer monitor at 4k would be an improvement.
Makes sense for TVs, I doubt many people sit 5 feet away from their 60 inch TV.