It depends on the TV, Blu-ray player, and the Blu-ray disc itself. Generally, Blu-rays look better on 4K TVs, but this can vary depending on the quality of the TV and Blu-ray player. DVDs upscaled to 4K can look good, but this also depends on the quality of the DVD and the TV's upscaling capabilities. Reviewers have found that the majority of content watched on 4K TVs is 1080p, so it is expected that Blu-rays would look good on these screens. 4K Blu-rays have superior video and audio quality compared to regular Blu-rays, with four times the resolution and enhanced color and contrast. They also have more storage space, allowing for more features. If you want to see your 4K TV's best picture, it is recommended to use a 4K Blu-ray player.