Some Pixel 4 users have found that factory resetting the phone will solve the issue
Frankly, we would have liked to have seen quicker action from Google considering that more than just a handful of Pixel 4 users resorted to the dreaded factory reset to get their Face Unlock working again. And it isn’t as though Google was caught unaware. Apparently this issue started with the November security update and the very first post about this problem found on Google’s own Pixel phone support page is dated November 24th.
If you need to contact Google about this matter, if you can unlock your phone go to Settings > Tips & support > Chat or Phone. Type in your information and then hit Send. If you have access to Twitter, you can send a tweet about your Pixel 4 to @MadeByGoogle. If you don’t want to factory reset your phone (and frankly we don’t blame you), you need to make your voice heard.