When face unlocks were first introduced, people would just hold the phone in front of your face while you were asleep and voila! They would have access to all your private information. However, tech companies soon realized this issue and made sure that your eyes were wide open when you were unlocking your phone using a face lock.
Source: Pocket Lint
Despite this, Google never considered it and received heavy criticism for its feature that allowed people to unlock your phone while you were asleep or not looking directly at the sensor.
Source: Android Central
But now they have come up with an update for Pixel 4 so congratulations to all the users because now your phone would not be unlocked unless your eyes are open and facing the sensor. If you're sure that people wouldn't try any such thing, you can always go and disable it in Pixel 4‘s face unlock settings but it is better to be safe than sorry.
Apple rolled out this feature in 2017 and was one of the first companies to realize the importance of “require eyes to be open” factor for face unlocks.
Along with this update, your Pixel 4 will also see improvement in Bluetooth audio input issues it was facing during calls.