On smaller screens, around 55 inches or less, the difference between 1080p and 4K may not be highly noticeable, especially at typical viewing distances. The human eye has a hard time discerning the extra detail and sharpness that 4K provides on these smaller displays. However, on larger screens, 65 inches and up, the difference becomes much more apparent. The increased pixel density of 4K results in a noticeably more detailed and crisper image compared to 1080p, even when viewed on a non-4K display.
That said, modern 4K displays are excellent at upscaling 1080p content to fill the 4K resolution. Through advanced image processing algorithms, they can enhance the sharpness and clarity of the 1080p image, bringing it closer to the quality of native 4K content. So while you may not get the full 4K experience, you'll still see some improvements in image quality when viewing 1080p content on a 4K screen, especially on the larger sizes.