HD content will not necessarily look blurry or poor on a 4K TV. In fact, most cable, satellite, streaming, gaming, Blu-ray, and other video content is 1080p or 720p, which is lower resolution than 4K. However, 4K TVs have a higher pixel count than 1080p TVs, which means they can display more detail and produce a clearer, more lifelike image. 4K TVs can also upscale lower resolution content to 4K, which can improve the picture quality.
However, there are some limitations to upscaling. While it can make lower resolution content look better on a 4K TV, it is not a perfect process and the resulting image may not be as clear or detailed as native 4K content. Additionally, some 4K TVs may have issues with HDR (High Dynamic Range) content, which can cause the picture to look too dark or washed out. To get the most out of a 4K TV, it is best to watch native 4K content whenever possible.