Apple's Face ID system, for instance, won't work if the user has their eyes closed by default and are actively looking at the display. Face Unlock working with the eyes closed isn't a flaw with the Pixel 4 and Pixel 4 XL. Google designed the facial detection system to work purposefully with eyes closed. The issue with having a Face Unlock system that works with the eyes closed is that it would allow someone to unlock your device while you are sleeping or unconscious.
Interestingly, the BBC reports that past leaks showed the Face Unlock system of the Pixel 4 had a setting in the menu that was to force the system to unlock only if the eyes were open. That setting is reportedly missing in the production devices the publication received for testing. BBC's Chris Fox posted a video to Twitter, embedded here, that shows the Face Unlock system in action with eyes closed.
Google has stated that Pixel 4 Face Unlock meets the security requirements as a strong biometric. Pixel product manager Sherry Lin noted that there are only two face authorization systems that meet the requirements for use for payments. Those include the Pixel 4 system and Apple's Face ID system.
Google recommends users who are concerned about the security of Face Unlock can use "lockdown" mode that deactivates facial recognition. The Pixel 4 and Pixel 4 XL devices were unveiled on October 15th and can be pre-ordered now.