When you watch 1080p content on a 4K TV, the TV's internal video processing is used to upscale the lower resolution signal to fill the higher resolution panel. This can result in a blurry or softened image because the TV is essentially creating additional pixels to fill in the gaps between the original 1080p pixels. This process is not as sharp or clear as if the content was originally filmed in 4K, which is why the picture may appear blurry.
Another reason for a blurry 4K TV picture is if you're feeding the TV a lower resolution signal than its native 4K capability. For example, if your Blu-ray player or cable box is still set to HD, the content will look blurry when upscaled to the higher resolution panel of your 4K TV. To fix this, you should make sure that you're using a device that supports 4K videos and using high-quality HDMI cables to connect your devices to the TV. Additionally, you can try adjusting the picture settings on your 4K TV to improve the clarity of the image.