When it comes to jean, john, sean and so on i don't feel there's a big problem if a John is followed by a Jean and they are both numbered nr. 1. That's my opinion.
And dukes are commonly numbered by the total number of dukes that's held the title before.
http://en.wikipedia.org/wiki/George_Spencer,_4th_Duke_of_Marlborough
Example of numbering dukes
No, dukes were commonly numbered just as kings (and counts)
Your example is from the English Peerage, and shouldn't be confused which what is considered 'commonly'
Also this George you list here isn't named George IV, but as George the 4th duke of Marlborough. In case of George IV it is part of his name in case of George 4th duke it is part of his title.
EDIT
Anyway your solution seems to be the easiest to make for the game.
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