HD content can look better on a 4K TV, but it depends on several factors. When you watch regular HD content on a 4K TV, the TV will upscale the content to fit the higher resolution of the screen. This process involves the TV's image processor analyzing the HD content and creating additional pixels to fill in the gaps and make the image look better on the 4K screen. However, the quality of the upscaling can vary depending on the quality of the TV's image processor and the original resolution of the content. If the upscaling is done well, the HD content can look much better on a 4K TV, with greater detail and clarity. However, if the upscaling is not done well, the image can look blurry or pixelated.
It's also worth noting that not all HD content will look better on a 4K TV. Some HD content may have been filmed or mastered at a lower resolution, and in these cases, upscaling may not be able to improve the image quality significantly. Additionally, if you are watching HD content that has been compressed or degraded in some way, the image quality may not be as good as it would be on a lower resolution screen. Overall, while HD content can look better on a 4K TV, the actual improvement will depend on the quality of the content and the TV's image processing capabilities.