Depends on your monitor size. Most ppl with 24" monitors are using 1080p on a desk. About 3ft away from their face, and it looks good. Moving upto a 27-28" at 1080p, you have the exact same amount of pixels filling a larger area, bigger pixels. So it starts getting grainy. Sorta like if you sit too close to the TV in the living room it looks bad, but back up to 8ft or more and it clears up. So moving to a 27"-28", most will also move to 1440p, more pixels, smaller pixels, looks a lot better at the same range, but the screen is also larger. 4k is an oddball, it has so many pixels, it looks good at any size or distance, be it 24" at 3ft or 50" at 8ft.
Its more than just trying to get a better picture, gotta take your normal viewing distance, screen type (TN/VA/IPS), pc ability, fps needs etc into consideration. You may end up looking at the ultra wides too, that'll depend also on what's good for you and your setup.