The reason why 4K videos look better on HD screens is due to the higher resolution and improved detail that 4K provides. 4K resolution, also known as Ultra High Definition (UHD), has a resolution of 3840 x 2160 pixels, which is four times the resolution of 1080p (1920 x 1080 pixels). This means that 4K videos have a much higher level of detail and a larger color gamut than 1080p videos, resulting in a sharper and more vibrant image.
When watching 4K videos on a 1080p monitor, the video is downscaled to fit the lower resolution of the monitor. However, the higher resolution of the 4K video provides more source material for the downscaling process, resulting in a sharper and more detailed image compared to a native 1080p video. Additionally, many 4K TVs use a process called upscaling or upconverting to improve the picture quality of lower resolution content, such as 1080p videos. This process uses algorithms to fill in missing details and create a more detailed image, resulting in a better overall picture quality.
In summary, the improved visual quality of 4K videos on HD screens is due to the higher resolution and improved detail provided by 4K, as well as the use of upscaling algorithms in 4K TVs to improve the picture quality of lower resolution content.