tl;dr
Joanna la Beltraneja remains Henry IV's heir, and her marriage to Alfonso V results in a personal union of Portugal and Castile. What happens to Columbus and the Americas (whatever they end up being named)?
The whole story:
Let's suppose, however unlikely, that Henry IV of Castile manages to gather noble support for Joanna as his heir. She is married to Alfonso V of Portugal. Portugal and Castile enters into a personal union similar to OTL union of Castile and Aragon. Let's further suppose that the Castilian nobility is not dominated and is treated reasonably well such that they are not in open revolt. Isabella of Castile is still married to Ferdinand of Aragon, but has renounced an claim to Castile.
How does this affect the discovery and colonization of the *Americas? In particular, what does Columbus do?
OTL Portugal controlled the circumafrican route to the Indies. Columbus was able to sell to Isabella and Ferdinand his expedition to the west as an alternate route to the Indies. It seems to me that ATL Castile will lack the motivation to such for this route since it is united with Portugal. (Or would the Castilian interests be excluded from the circumafrican route, and some nobles might finance a search for alternatives?)
Could Columbus still convince Isabella and Ferdinand? I doubt this but do not completely rule it out. Aragon is primarily a Mediterranean power and lacks direct access to the Atlantic. Are there ways around this limitation? Perhaps somehow seizing Tangier?
Could Columbus find support in France or England? This has often been explored before on this site, but feel free to defend the idea again. What route would he take? OTL the Castilian Canaries and the trade winds made for a fairly easy choice for the initial western course. If Columbus sails for France or England, does he take a different coarse?
And to head into the highly improbable, but not quite impossible, could Columbus be so focused on the western route that he might attempt to sell the idea to Morocco?
Or is Columbus just out of luck?
Of course, Europeans will eventually encounter the *Americas whether by Volta du Mar, Basque and Breton fishermen, or something else. But for now, let's focus on a deliberate effort to cross the Western Ocean.