The sources over at XDA Developers dug through the source of the newest Android release, which Google made public the other day, and found several intriguing commits relating to display refresh rate. One of these describes content-based fps detection, while another describes the implementation of an overlay that will show when the device is running at 60Hz vs 90Hz.
Now these in themselves wouldn't be proof that they're related to the Pixel 4 in any way since Google constantly adds things to the AOSP code that accommodate the growing number of devices from many different OEMs. However, a comment left behind in a commit that has since been removed (but can still be viewed by going through the history) makes things quite clear.
Here's what it's about. The code in question adds an enable/disable flag to toggle the 90Hz mode, and it's only intended to be used temporarily until the proper solution is implemented (which happened not long afterwards). But the main takeaway is this: "The switch in settings should only be available to P19 devices". P for Pixel, 19 for the year... and you get the point.
Through some more digging, it became clear that Google intends to expose the refresh rate overlay as a Developer Option. The overlay will be shown under the clock in the status bar, as a rectangle that's red for 60Hz and green for 90Hz. This will be handy if you ever want to check whether an app is running at 90Hz or not. Finally, Google also wants to detect when a video is playing so the Pixel 4 can automatically adjust the refresh rate.