A month ago, Google explained why it doesn't support 4K video recording at 60fps on its latest flagships (or any of its phones, for that matter): It said the majority of users stick with 1080p, so it'd rather focus on improving that mode and avoid large storage consumption by 4K 60fps videos. As it turns out now, there might be another reason related to the Pixel 4's Snapdragon 855 processor, which always pulls images from the two rear cameras at once.
Mishaal Rahman and his team at XDA Developers managed to unlock the 4K 60fps mode in Google Camera on a rooted Pixel 4. They quickly noticed that recorded videos never exceeded 38fps though, which isn't ideal.
GCam modder savitar/defcomg said the Pixel 4 connects to both cameras at once in all modes, likely to allow for seamless switching betwen lenses when zooming. Because of this, the ISP in the 855 can't handle recording 4K60 on the main and 1080p60 on the secondary.
— Mishaal Rahman (@MishaalRahman) November 8, 2019
The team found out that the Pixel connects to both rear-facing cameras at once, which allows you to switch between the two seamlessly while zooming. It's also the reason why the Snapdragon 855 can't handle recording 60fps in 4K: It doesn't have enough bandwidth in its image sensor processor to maintain the high frame rate on two lenses at once.
I *knew* it was related to performance, ha.
Great sleuthing work, as always @MishaalRahman.
The solution seems obvious - for 4K60, don't pull from both cameras at the same time.
— Artem Russakovskii (@ArtemR) November 8, 2019
The solution seems simple enough: To enable 4K 60fps recordings, Google could simply disable pulling from both cameras at once. This would break optical zoom but could enable a feature that'd make 20 percent of users much happier.