The new sighting comes courtesy of 9to5Google but doesn't give us much in the way of new information about the device. We can see the rear camera array, believed to be a dual-lens affair with a flash and spectral sensor (for sensing gestures) as well.
Spotted on the London Underground, the phone is hidden away in a generic-looking case, so we don't get a look at the front of the device or the software (which we're thinking is going to be Android Q).
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Even without much in the way of new details, it's another sighting to file away ahead of the phone's launch. The consensus seems to be that Google will launch the phone in October, though that's yet to be confirmed.
Last month Google took the unusual step of leaking an early shot of the Pixel 4 itself, amidst a flurry of leaks from unofficial sources. It shows a chunky rectangular rear camera array on the back of the phone.
Other leaks have suggested the front of the phone will feature a larger top bezel rather than a notch for fitting in the front-facing camera components, while we're again expecting a standard-sized phone and an XL variant.
The screen sizes of those phones could go up to 5.8 inches for the Pixel 4 and 6.4 inches for the Pixel 4 XL, which would make them slightly bigger than the Pixel 3 flagship phones that got launched in 2018.
Specs-wise, the Pixel 4 is being tipped to come with a Snapdragon 855 processor inside and 6GB of RAM. Chances are we'll see a few more shots of Pixel 4 prototypes being carried around between now and October.
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Dave is a freelance tech journalist who has been writing about gadgets, apps and the web for more than two decades. Based out of Stockport, England, on TechRadar you'll find him covering news, features and reviews, particularly for phones, tablets and wearables. Working to ensure our breaking news coverage is the best in the business over weekends, David also has bylines at Gizmodo, T3, PopSci and a few other places besides, as well as being many years editing the likes of PC Explorer and The Hardware Handbook.