Well, it is unsupported Beta software, so a lot of it isn't even finished from what I can tell. It's not recommended for a live site. Why can't you go back to vB4, exactly? Do you not still have a backup of vB4? (Granted, you'll loose any posts made since upgrading.)
One method I've never tested, but figured should work, was to migrate to one of those free forums available that support vB migration. Then migrate back to vB on whatever older version you have. Time consuming, and again, I've never tested it. But theoretically, it should work.
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