Will https://old.reddit.com/r/Pixel4a/comments/1ic55ki/succesfull... help you undo what was done? It links to Android 13 S1 version August 5, 2023.

I was fortunately able to catch my 4a after it had downloaded the update but still needed a reboot to install. Followed https://old.reddit.com/r/Pixel4a/comments/1i0zih0/the_only_w... , installing NextDNS to prevent the update from downloading.