Watching a 4K video on an HD (1080p) screen will result in the 4K video being downscaled or resized to fit the screen's lower resolution. This means that the 4K video's higher level of detail and clarity won't be fully displayed on the 1080p screen.
However, even with this downscaling, the 4K video can still provide a noticeable improvement in picture quality compared to a native 1080p video. The 4K source material has more information and a higher dynamic range, which can translate to better color accuracy, contrast, and sharpness when scaled down to 1080p. So while you won't see the full 4K experience, you may still notice a visual upgrade compared to a regular 1080p video.