In a nutshell (tl;dr)
Full HD on a 4K monitor looks worse than on a Full-HD monitor.
This happens due to blur always added by monitors and graphics drivers when scaling, though at integer scaling ratios (e. g. 2 in case of FHD→4K) it could be avoided.
As a result, sharpness in 3D games is decreased, white lines are gray, and pixel art is not pixel art anymore.
Each logical pixel could be displayed as a square group of integer (2×2, 3×3) number of physical pixels of the same color.
Such lossless scaling could be built into graphics drivers, but this is still not done.
- Sign the petition for nonblurry scaling in graphics drivers.
- Vote for adding the feature to the AMD graphics driver.
- Express your support in forums: GeForce, AMD, Intel.
- Complain of blur to technical support of your GPU’s vendor: nVidia, AMD, Intel (1, 2).
- Complain of blur to technical support of your monitor’s manufacturer.
- Request the feature for game engines Unity, CryEngine.
- Tell about the issue to as many people as possible.