Since Vista, Windows has gradually improved the HiDPI compatibility of its OS and development platforms (multi-monitor compatibility was introduced with Windows 8.1). However, there are still many applications that are not or not fully compatible for the following reasons. In these cases, OS forcibly enlarges/reduces an application created in the previously common way, making it blurry. Also, some applications are displayed without blurring, but the display size is reduced.
The conclusion should be that beyond a certain point, a physically smaller display will require physically fewer pixels to remain usable at HiDPI (assuming similar viewing distances). Fitting "4K" pixels into such a small space is worse than simply using fewer pixels, even if you do not use the marketing-friendly term 4K screen resolution. Pixel density is the specification that defines how detailed a display is. It is expressed in units of DPI (Dots Per Inch) or PPI (Pixels Per Inch).
