Actually, the proyects of division of Spanish Empire into three or four kingdoms with spanish princes in the late eighteenth and early nineteenth were a Commonwealth proyects in themselves
I've always been skeptical of this idea. To my mind, just because the Portuguese pulled off the idea of royalty in America, a lot of posters here thus seem to think that anyone could pull it off. I think instead that one of two things would happen if the Spanish tried to pull off splitting up their inheritance into four. Either the princes who were to become Kings would refuse to travel to their new kingdoms or if forced they would become disgruntled and would abdicate/encourage revolution by their apathy. To put it simply, up until after the age of Empire had passed anyway, simply put the Americas (outside of the USA if you are so inclined to argue) were not a prestigious place to live - whether they were actually a nice place to live or not, they had the reputation for being cheap, disease-ridden, low-class backwaters, and undeniably they had no sense of upper class high society that Europe enjoyed. Take for example when the Mexicans shortly after their revolution attempted to find an Austrian to become Emperor of Mexico.
Everyone refused. They had no interest in moving to America - a huge country and a throne to themselves just didn't compare to fashion, balls, wealth, and comfort. And no, they couldn't implant high society - any nobles they attempted to bring with them would refuse to travel too. The reason it worked for the Portuguese is the sole reason that, with Napoleon on the march, the Portuguese royalty faced being taken into French custody and stripped permanently of their land and privilege.
Not to mention...why would the Spanish split up the Empire into four? If they tried doing it when discontent was rising and revolution was seeming certain, they would just be sending the princes to a lynch mobbing which would probably encourage revolution across the Empire even more - not to mention it would be a tacit admission that they didn't believe that they could win against a revolution - they might have been correct, but that's an admission you can't afford to make. It would be a bit like the UK in 1776 saying
"OK, we put our hands up. We made a mistake and we can't win. We're going to pull out of North America without even trying now, and if anyone attempts to threaten us in our moment of weakness then so help us God we will do our best to capitulate humiliatingly against you too." It just wouldn't happen. Avoiding a fight is something that you are praised for on the school playground. It is an unacceptable loss of prestige if you are a world-spanning Empire, no matter how bad the fall will be when you lose.
On the other hand, if they tried sending princes to become Kings when there were barely any murmurs of discontent in the colonies...well why on earth should they? It may be a disastrously poor way to conduct internal policy to concede defeat to internal rebels without a fight, but far far far worse is to concede defeat to the possibility that there might be rebels. Simply put, it was the sole reason for existence for every Empire (on the macro-politics level) to remain whole and become as powerful as they could be. Dividing your Empire up is something that you only do if you are losing a talented ruler (i.e. Alexander the Great) and you know the Empire can't hold together. The Spanish Empire had held together for 300 years of dross Kings. There was no logical reason it should commit seppuku for the sake of making life a bit easier for local government. It's just a nonsense move.
Not intending to criticise your opinions, of course. I just want to set the record straight as to how and why decisions like these are/were made. You can't judge ancient Empires by modern-day standards of decision making and values. You just can't.