The final rank of Grand Duke or Duchess, however, is only achieved after the player has defeated all regular noble Trainers at least once (excluding Gym Leaders, the Elite Four, and Diantha, that is).
So yes, it is possible to obtain the rank of Grand Duke/Dutchess before defeating the Elite Four. All other ranks are obtained by passing a certain number of points required, the points being obtained by defeating trainers in the Battle Chateau. You can see the required points and the amount of points given for certain trainers in the source.
Source
Hope I helped
