The Google Pixel 4's 90 Hz Smooth Display feature seems to be dependent on the display brightness setting and not on the 'content' as was originally claimed by Google. This was first discovered by a Redditor and subsequently confirmed by other reviewers as well. Although there is a way to force 90 Hz in the Developer settings, by default, the Pixel 4's screen can refresh at 90 Hz only if the brightness slider is set above the 75% mark.
https://www.notebookcheck.net/Cheeky-Google-The-Pixel-4-throttles-the-display-refresh-rate-based-on-brightness-setting-and-you-wouldn-t-even-know-it.439984.0.html