When a heir accedes to the British throne, they may choose the name they will rule under, and it does not necessarily have to be their birth name. There is no guarantee that when William takes the throne in real life that he will, in fact, choose to be William V.
The monarchs are listed in rough chronological order by the time of their coronation. Please note that this template only uses information from DWU sources. Real world information should not enter into this.