One of the major new features of the Pixel 4 is the 90Hz refresh rate for its display. However, this 90Hz "Smooth Display" feature isn't always active by default, and now it's been discovered exactly what triggers it.
The Pixel 4's 90Hz refresh rate comes on when the screen brightness is above 75%. This was first discovered by Reddit user OrganicNebula and confirmed by XDA-Developers' Mishaal Rahman. This means that, when your screen brightness falls below 75%, your screen will switch to a 60Hz refresh rate.
The 75% screen brightness level is pretty high, and so it's likely that you're only going to get that high brightness and 90Hz refresh rate in super bright environments, like when you're outdoors.
It's worth noting that it is possible to force your Pixel 4 to stick to the 90Hz refresh rate all the time. There's a toggle in the phone's developer settings called "Force 90 Hz refresh rate" that does exactly what it sounds like. Google does note, though, that enabling this feature will increase your phone's battery usage.
There are other Android phones on the market that have a 90Hz refresh rate that's active all the time, and so some Pixel 4 owners may be frustrated that their new flagship phone doesn't do the same. Google hasn't offered an official explanation about its decision to tie the Pixel 4's refresh rate to screen brightness, but it seems like the company was concerned that enabling it all the time could cause too big of a hit to battery life, which is why it notes that the toggle to force 90Hz refresh rate all the time will increase battery usage.