In a nutshell: When Google revealed its latest line of Pixel phones---the Pixel 4 and Pixel 4 XL---it confirmed a feature that had been rumored for some time: the 90Hz refresh rates. But it's been discovered that users aren't going to get the full benefit if their display's brightness is set too low.
The discovery was made by Redditors, who found that the Pixel 4s' 90Hz Smooth Display drops to 60Hz, which is the standard for most smartphones, when the brightness is set below 75 percent.
Some other handsets, such as the OnePlus 7T and 7T Pro, also have 90Hz refresh rates, though these aren't tied to the phones' brightness levels. Google did say that it will automatically switch the refresh rate of the Pixel 4 "for some content."
It seems that Google included this limitation to help improve the battery life of the Pixel 4, which has come in for some criticism. But droidlife writes that the automatic switching can be overridden by going into the Developer Options menu and enabling the Force 90Hz setting.
A brightness of 75 percent is more than most people will use in an indoor environment, so although you might be getting more battery life from your Pixel 4, you won't be enjoying the Smooth Display feature.
Interestingly, one Twitter user discovered that if Ambient EQ is enabled and the ambient light is very bright, the 90Hz refresh will stay enabled even when the brightness is turned all the way down.