In light of all these varying HiDPI display compatibilities, the development of applications and the changes in monitor pixel density have been under great speculation. Until recently most monitors typically had a pixel density * of 96 ppi. Various different company's monitors were designed to match the display size (inch size) with the resolution, therefore most OS and applications were also made based on this 96 dpi standard.
4K is a (not sanely standardized) description of a number of physical pixels. It's an approximation of the number of pixels wide something is (as opposed to the previous convention of how many pixels tall as in 480p, 720p, 1080p, etc.). Something with somewhere around 4000 pixels wide will be called 4K. So you have a variety starting from 3840 pixels up to at least 5120 pixels on “ultrawide 4K” displays, broadcasts, or video.
Ever since Vista, Windows has been gradually advancing the HiDPI compatibility of its OS and development platforms (multi-monitor compatibility was introduced from Windows 8.1). However there are still many applications that are not compatible or fully compatible due to the following reasons. In these cases, an application created in the previously standard way will be forcibly enlarged / shrunk by the OS, causing it to become blurry. Furthermore some applications may display without blurring, but the display size will be reduced.
The takeaway should be that at a certain point, a physically smaller display requires physically fewer pixels to remain usable at HiDPI (assuming similar view distances). Cramming “4K” pixels into that small of a space is worse than just using fewer pixels, even if you don't get the marketing-friendly 4K term.
Depending on the application's development platform
HiDPI compatibility may not be facilitated throughout the entire application.
The number of device pixels that make up a CSS pixel in one direction is its Device Pixel Ratio (DPR). You can interpret this as the width (or height) of the grid of device pixels that fit inside one CSS pixel. Every device has a different DPR. Higher resolution devices have a higher DPR. These devices can see sharper images because they devote more screen pixels to each CSS pixels. This means nuances in the image are better represented.If a Web browser (or any other application for that matter) naively continues to say that one pixel according to the app's concept of pixels is the same as one pixel on the screen, then eventually you have text and images so small that they're impossible to view easily.
PPI stands for "pixels per inch" (not per square inch). One inch is 2.54 centimeters. If you reduce the pixel height without changing the screen size of the LCD screen, the ppi will increase and the higher this value, the higher the screen resolution.
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james west