A regular Blu-ray player cannot play 4K Ultra HD discs. 4K Ultra HD discs are encoded with a different video codec (HEVC) and have a higher resolution than regular Blu-ray discs. Regular Blu-ray players do not have the necessary hardware to decode the HEVC codec or display the higher resolution, so they will not be able to play 4K Ultra HD discs.
To play 4K Ultra HD discs, you will need a dedicated 4K player. These players have the necessary hardware to decode the HEVC codec and display the higher resolution. Some 4K players can also upscale regular Blu-ray discs to near 4K quality, but the picture will not be as sharp as a native 4K Ultra HD disc. Additionally, some 4K players can also play standard Blu-ray discs, but they will not be able to upscale them to 4K quality.