According to XDA, the pixel 4 and pixel 4 XL screens will automatically switch between 90hz and 60Hz based on brightness last month. Mishaal Rahman, chief editor of XDA, found that when the brightness is lower than 77%, pixel 4 will automatically reduce to 60Hz. Although users can force the 90hz refresh rate on in the developer option, this is not the ideal solution because the endurance will be affected.
After the news was exposed, Google released a statement that it will maintain a 90 Hz refresh rate under more brightness conditions through subsequent updates, which will be packaged and pushed together with the November 2019 security patch.
The XDA points out that pixel 4 and pixel 4XL have different performances after updating. When the brightness of the display is lower than 42%, pixel 4 will switch to 60Hz refresh rate, while pixel 4XL will keep 90hz unchanged.
It is reported that the restriction of pixel 4 to use 90hz may be due to the concern of battery life, which is only 2800mah. In contrast, the pixel 4XL battery has a capacity of 3700mah.