Vanity License Plate - 4 OF 5

So many possibilities here. One of these has a good chance of being right (but all other suggestions welcome)

80% there?
Personal rating system ... being brutally honest in self-assessment?
4th of 5th family member?
4th of 5th child in the family?
4th of 5th car in the family?
4th of 5th BMW in the family? (If so, do you adopt?)

Or they could be a fan of Star Trek ...
remember the Borg, "7 of 9"?