Non-4k 1080p movies and shows can look good on a 4k TV, but it depends on a few factors. The first factor is the quality of the upscaling process. Most 4k TVs have built-in upscaling technology that can convert lower-resolution content to a 4k resolution. The quality of the upscaling process can vary between TV brands and models. A good upscaling algorithm can make the picture look sharper and more detailed, while a poor one can result in a blurry or pixelated image.
Another factor that affects the picture quality of non-4k content on a 4k TV is the size of the screen and the distance from which you are viewing it. The higher pixel density of a 4k TV means that the benefits of the higher resolution become more apparent on larger screen sizes. On smaller screens, it can be difficult to discern the additional fine details in 4k. As a general guideline, the difference between 1080p and 4k is moderately discernible on screens between 50-55 inches.
In summary, non-4k 1080p movies and shows can look good on a 4k TV, but the quality of the upscaling process and the size of the screen can affect the picture quality. A good upscaling algorithm and a larger screen size can make the picture look sharper and more detailed, while a poor upscaling algorithm or a smaller screen size can result in a blurry or pixelated image.