Pixel 4 display has a flaw, yet again! An Android developer on reddit reports about it. Yes, this is all about that buttery smooth 90Hz display. Once you dim the display up to enough level, the 90Hz turns to 60Hz. And this happens with both Pixel 4 and Pixel 4 XL.
Android developer Brian Sefcik tells about this flaw in the Pixel. Once you dim the display to 75%, the OLED panel is forced to underclock to 60Hz from 90Hz. In fact, Mishaal Rahman from XDA confirms this using the Logcat tool over ADB, which comes with Android SDK.
It might be the thing that Google might have considered that if you dim the brightness, you may not do anything to benefit the 90Hz refresh rate. Also, do remember that the 90Hz panel eats more battery than the conventional 60Hz refresh rate panel.
You can just flip the switch in the Developers Options in the section called Force 90Hz Refresh Rate. This will force the Pixel 4 display to use the 90Hz refresh rate even on dimmer light. But remember, it may eat the battery faster than ever, and with that puny size battery on Pixel 4, you might have to run for the wall socket again.