Hello,
so, just to be sure: you attempted to upgrade to version 4 of Joomla, but you had issues. So you want to rollback to Joomla 3, by restoring a previous backup archive, am I right?
If that's correct, the best thing to do is to clean up your website, by deleting all the files. This because between version 3 and version 4 several files have been moved around, have been added or renamed. During the restoration, those extra files are not deleted, so you end up with conflicts. The best way is to start from a clean state and then restore your backup archive.
PLEASE NOTE! Before deleting everything, triple check that your backup archive is actually working fine, by restoring it somewhere else (locally or in a subfolder). Then remember that the backup folder of Akeeba Backup IS NOT included inside your backup archive. This means that you have to download them manually before deleting everything, otherwise you'll end up with only one archive.
Davide Tampellini
Developer and Support Staff
🇮🇹Italian: native 🇬🇧English: good • 🕐 My time zone is Europe / Rome (UTC +1)
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!