If you go to your hosting control panel you should have a link to phpmyadmin where you can administer your database just view the database and view the pp_settings table and scroll down to setting 84 and see what version is typed in there for the setting
What I am saying about thumbs is if you view your gallery thumbnails and you have an address like this
http://www.domain.com/gallery/data/500/thumbs/image.JPG
then you can run 5.0 forward once as you have at least a 5.0 version forward. Note there are thumbs directories under each data directory.