I bought a used 4th generation ipod that only has IOS 6. Can I download an upgrade somehow to at least IOS 9?>

If you go into setting>General>Software Update> there should be an option. If there is other updates see if you can just skip those and go straight to iOS 9. I tried skipping from iOS 14 to 16 on my iPhone 11 but the update kept freezing so if you have that issue try doing the other updates. It may take a while and make sure you have a backup if there are any songs on there you want to keep.

The last update for a 4th generation iPod touch was iOS 6.1.6. It cannot be updated beyond that.