The move was announced by Google after early testers of the new handset were able to use face unlock with eyes closed, prompting fears that it could lead to unauthorised access to the device.
It was also suggested that someone else could get into the phone by holding it in front of the face of its sleeping owner.
In response, Google said the feature met security requirements and stated that it would “improve face unlock over time”.
The tech company now says it is working on an option for users to require their eyes to be open to unlock the phone, which will be delivered in a software update “in the coming months”.
A Google spokesman said: “In the meantime, if any Pixel 4 users are concerned that someone may take their phone and try to unlock it while their eyes are closed, they can activate a security feature that requires a pin, pattern or password for the next unlock.
“Pixel 4 face unlock meets the security requirements as a strong biometric, and can be used for payments and app authentication, including banking apps.
“It is resilient against invalid unlock attempts via other means, like with masks.”
The Pixel 4 is the first Google smartphone to feature face unlocking security measures, following the likes of Apple which introduced the feature to the iPhone in 2017.
Apple’s Face ID unlocking system requires the user’s eyes to be open and for them to look directly into the camera to access the device.
The Pixel 4, which goes on sale in the UK on 24 October, features Motion Sense, which lets the user do things such as change music tracks and snooze alarms without having to touch the device. It also has an Adaptive Battery which learns the user’s favourite apps and reduces power to those that are rarely used.
The model follows the budget-friendly release of the Pixel 3a – which at £399 is aimed at consumers turned off by more expensive devices.
The Pixel 4 and 4 XL have two rear cameras and will retail at $799 (£669) for the 64GB version and $899 (£829) for the larger XL.
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