Hi, (soon to be ex) Pixel 4a user here. Regardless of why this update has happened, the way Google have went about this update is sketchy at best, and deceitful at worst. To be plain: this phone has been EOL for 1½ years now. This update has appeared out of the blue and specifically decimates the battery + charging capabilities. My most charitable view is that Google have found a major fault with the Pixel 4a battery…
My wife was using a pixel 4 until last year and upgraded to a 7, I took her old phone and switched it over to my info and used it for like 2 days before it got a random update and refused to ever work again. Her 7 just got an update a couple of days ago and the battery went dead and it refused to boot until it'd been plugged in for an hour and then would only boot into recovery mode before finally returning to normal. I think google just has something really screwed up with their update process.