Google Camera 4.4 surfaced recently in a Developer Preview build of Android O. Now, this app is finally available to the public via Google Play, although only in select markets. Obviously, the US is one of them.
The previous version introduced front-facing selfie flash (thanks to a full white display that flashes right before taking the shot), and this one comes with a few new features and changes, most of them quite hard to notice. First, there is the double-tap to zoom capability, then a slightly enlarged shutter button, as well as a toggle button for photo/video mode.
At last, we should also mention that the "Camera sounds" option that arrived with Google Camera 4.3 is now called "Camera shutter sounds." Unfortunately, versions after 4.2 do not offer 32-bit support, so those who use handsets with chips pre-dating the Qualcomm Snapdragon 808/810 cannot use Google Camera 4.3 or 4.4.