Two cool new features of the Pixel 4 camera will not be ported back to older Pixel phones - Live HDR+ and Dual Exposure Controls. Those can only run on the new hardware, so the Pixel 3, 3a and others are out of luck. To be fair, few phones are fast enough to do real-time HDR preview. The Pixel 4 and 4 XL technically aren't either, what you see on their viewfinders is an approximation of the final result. The effects of the Dual Exposure sliders are previewed in real time so they aren't supported on older models either. Realize your artistic vision by adjusting brightness and shadows...