Some Pixel 4 users have found that factory resetting the phone will solve the issue
As one Redditor wrote just a few days after the ball dropped in Times Square, "I have this too! Had a message to re-enroll my face data, despite face unlock working. Now the enrollment will appear to work, but my face won't unlock my phone. I thought I might have a hardware fault, but given that multiple of us have had this issue occur at the same time, perhaps it's a bug or back-end fault? I'd factory reset my phone, but first I'll wait and see if the problem is fixed by the time of the January patch on Monday." Here's a spoiler though, factory resetting the phone solved the problem according to those who ended up at the point of no return after the January update failed to deliver the answer.

Some Pixel 4 users have found that Face Unlock has been broken
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.
What some other Pixel 4 owners are waiting for is a promised update from Google that will include a setting allowing them to demand that their eyes be open for Face Unlock to work. As things currently stand, a bad actor who wants to get into your bank account could hit you over the head with a Pixel Slate and render you unconscious. Picking up your phone, he can use Face Unlock (unless, of course, its not working) and unlock the handset. Then he could use your fingerprints to open up your banking app and wipe you out. Sure, it's far fetched. But that doesn't mean that it can't or won't happen.
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