It is possible to use a regular Blu-ray player on a 4K TV, but the image quality may not be as good as it would be with a 4K Blu-ray player. Some Blu-ray players have upscaling capabilities that can make the image look better on a 4K TV, but this will vary depending on the player and the title being watched. The difference in image quality between a regular Blu-ray and a 4K Blu-ray on a 4K TV may be subtle and may be more noticeable on some titles than others. Overall, if you want the best possible image quality on a 4K TV, it is recommended to use a 4K Blu-ray player.