You can download the old versions here
Make sure after you downgrade you delete the data file for version 5 so that if you do decide to update in the future, the programs will be forced to do this from your 4.6.8 file again (Database.pediadata).
You will find the file(s) in the Pedia data folders, located in your Home folder under ~/Library/Application Support/. The files to delete are: Database.bookpd for Bookpedia, Database.dvdpd for DVDpedia, Database.cdpd for CDpedia and Database.gamepd for Gamepedia.
If you're on Lion, it hides the Library folder by default so to go to your Pedia data folders you have to use the Finder's 'Go' menu, select 'Go to Folder' and copy paste ~/Library/Application Support/ into the window that appears.