The World is home to home to ten 4-diamond lodgings: Animal Kingdom Lodge, Animal Kingdom Villas, Beach Club, Boardwalk Inn, Contemporary, Grand Floridian, Polynesian, Yacht Club, Swan, and Dolphin; but no 5-diamond's.
We raised the question of what would it take to make the Grand Floridian (for example) into a 5-diamond property. I think the better question is, why would Disney want to?
With rack rates consistently north of $500 per night, GF is already charging about what you would expect to pay for a 5-diamond resort (and even higher than some). Sure, upgraded decor and some extra staff might get the Grand Floridian that coveted fifth diamond. But is there any way for Disney to recoup the outlay? There simply isn't anything to be gained.