You can watch 4K movies on a 1080p laptop screen, but the quality will be affected. The 1080p resolution is lower than the 4K resolution, so the image will be scaled down to fit the lower resolution. This means that the image will not be as sharp or detailed as it would be on a 4K screen. Additionally, the 1080p screen may not be able to display the full range of colors and contrast offered by a 4K video, which can further affect the overall picture quality.
However, it's worth noting that the quality of the 4K video on a 1080p screen will also depend on the specific laptop and its graphics card. Some laptops may have a more powerful graphics card that can handle 4K video better than others, so the quality may vary. Additionally, if you plan to watch 4K videos on your laptop frequently, it may be worth considering connecting it to an external 4K monitor for a better viewing experience.