If you have a 4K TV and a 4K Blu-ray player, it's worth investing in 4K DVDs for the best picture quality. While your 4K TV and 4K Blu-ray player can upscale standard Blu-ray DVDs to 4K, the picture quality may not be as good as watching a native 4K DVD. Upscaling technology can only do so much to improve the picture quality, and it's not always able to match the quality of a native 4K source. Additionally, 4K DVDs often have higher bitrates and better color accuracy than upscaled Blu-ray discs, which can make a noticeable difference in picture quality.
It's worth noting that not all 4K TVs are created equal, and some may not be able to play 4K Ultra HD Blu-ray discs. If you have an older 4K TV, it may not have the necessary hardware to play 4K discs. In this case, you may need to purchase a new 4K TV or a 4K Blu-ray player that supports 4K Ultra HD Blu-ray discs. Additionally, some streaming services may not support 4K content, so it's important to check the specifications of your TV and streaming services to ensure that you're getting the best possible picture quality.