The problem that some have with the device is that face unlock will unlock the phone even if the user's eyes are closed. Not requiring a person's eyes to be open would mean that someone could pick up the phone and unlock it while the owner slept or while they were unconscious. Most face unlock-style features on devices, such as the iPhone line, require the eyes to be open (by default).
Face unlock functioning with the eyes closed isn't a flaw in the Pixel 4; that is how Google designed the device. Google has stated that it will be issuing a software update that adds in the option to force the face unlock feature to require the eyes to be open. Google has noted that Pixel 4 owners who are concerned about someone using face unlock to unlock their smartphone while their eyes are closed can use "lockdown" mode to thwart that.
Lockdown mode requires a pin, pattern, or password to unlock the device. Google notes in its statement that Pixel 4 face unlock meets the security requirements as a strong biometric and can be used for payments and app authentication. The search giant also noted that face unlock can't be fooled with masks.
Last week, we talked about the fact that early leaks had shown the toggle to require the eyes to be open for face unlock to work. That feature isn't on shipping devices or devices that have been loaned to reviewers.