Google announced several enhancements to the photo-taking experience on the Pixel 4, including a more powerful zoom and new astrophotography abilities. As time passes, more details are emerging about other smaller improvements coming to the device, like Top Shot being available for short videos, and now we're learning about a new feature called Frequent Faces.
Frequent Faces was previously discovered by XDA, who theorized that it could be a Pixel 4 exclusive. Now we have a confirmation that it'll indeed be so. When enabled under the cam's settings, it stores face data on the phone without sending it to Google.
The goal behind Frequent Faces is to help your phone focus and suggest better snaps for the people you often photograph. Google's wording on the matter is a little confusing though. It sounds as if the feature will only work after the fact, helping Top Shot recommend photos where your favorite people have their best poses (eyes open or funny facial expression, for example). So if your kid is posing with friends, the priority would be to get a Top Shot where he/she looks best, even if at the detriment of the other children.
It doesn't look like Frequent Faces will be effective while taking the picture, so you likely won't get much help with focusing or properly exposing your favorite people. Maybe that ability will come later.