For all Pixel phones beyond the Pixel 3 up to the Pixel 5, unlimited photos applies to High Quality only, and will continue indefinitely, even beyond the June 2021 deadline for other phones. Original Quality backups will count against your Google Drive allotment. Google hasn't yet been specific about phones beyond the Pixel 5, but most people think unlimited backups will no longer be a thing.
This applies only to photos backed up through Google Photos.
There will probably be modest sale prices for the 4a and 4a 5G when the 5a comes out, but I doubt there will be a permanent price drop until much later. Remember also that the 5a will only be released in the US and Japan.