It is generally agreed upon that Blu-rays played on a 4K player will not look better on a 1080p TV. This is because the 4K player will downscale the picture to fit the lower resolution of the 1080p TV, which can actually make the picture look worse than if you were to play the Blu-ray on a 1080p player. Additionally, 4K Blu-rays are mastered to take advantage of the higher resolution and HDR capabilities of 4K TVs, so they may not look as good on a 1080p TV even if the player is able to upscale the picture.
However, some people argue that standard Blu-rays can look better on a 4K TV than on a 1080p TV, due to the higher quality of the Blu-ray disc and the better upscaling capabilities of 4K players. Additionally, some 4K Blu-rays may include features such as HDR and wider color gamut that can enhance the picture quality, even on a 1080p TV. Ultimately, the quality of the picture will depend on a number of factors, including the quality of the Blu-ray disc, the player, and the TV, so it is difficult to make a blanket statement about whether Blu-rays played on a 4K player will look better on a 1080p TV.