Since there's no converter for vBulletin 5 to 4, it means that everything has to be moved manually from the database of vBulletin 5 to 4. However, since the structure of the database is different, it takes a lot more time to do this.
Your best bet would be to hire someone to do this. If you create a thread
here, you might be able to find someone who will do it for less money.