DVDs can look good on a 4K TV, but the quality may vary depending on the TV's upscaling capabilities and the quality of the DVD itself. Upscaling is the process of converting lower-resolution content to fit a higher-resolution screen, and it can sometimes introduce artifacts or softness to the image. Some people have reported that DVDs look better on their 4K TVs when they set the Blu-ray player to output the native resolution, rather than upscaling to 1080p or 4K. This suggests that the upscaling process can sometimes introduce artifacts that are not present in the original DVD.
However, some reviewers have found that certain DVDs, such as those from the Criterion Collection, can look excellent on a 4K TV. It's also worth noting that the quality of the DVD itself can make a difference in how well it holds up on a 4K TV. For example, a well-mastered DVD with a high bitrate may look better than a lower-quality DVD with a lower bitrate. Additionally, some TVs have better upscaling capabilities than others, so it's possible that a higher-end TV may produce better results than a lower-end model. Ultimately, the best way to determine how well a DVD will look on a 4K TV is to try it out for yourself and see how it looks to your eye.