If we assume Charles IV has a son (most likely option for names would be Louis, Philippe or Charles) and still dies on schedule, then the regency would likely go either to his mother Jeanne d'Evreux or his cousin Philippe de Valois (OTL Philippe VI). Queen mothers being regent was somewhat the tradition in France: Anne of Kiev was a regent for her son Philippe I, and then you have the case of Blanche de Castille during the minorityof Saint Louis, and there are also later examples. That being said Philippe de Valois is the closest male relative to Charles, so he probably would also have a shot. And as was mentionned earlier, we also had the case of the future Philippe V being regent during the pregnancy of Clemence de Hongrie and then for the five days John the Posthumous lived: so Philippe de Valois could claim the regency based on that precedent.
In the specific case of Blanche of France being born as a son though, it would be Philippe de Valois that would get the regency. For the simple reason that he already was at the time of Blanche's birth: Blanche was a posthumous daughter to Charles IV, so her mother Jeanne was still pregnant with the contemporaries having no way of knowing if the child would be a girl or a boy. A regency was thus declared and Philippe de Valois was declared regent: OTL his regency ended once Blanche was born as he became King, but here he probably would conserve it.
The Hundred Years' War would probably be butterflied away, in the sense that there won't be a confrontation between England and France over the succession to the French throne. However, England still holds Guyenne and that will necessarilly provoke conflicts between France and England over the control of it. Furthermore, England could also be tempted to help Flemish rebels, as Flanders did most of his commerce with England and was extremly rebelious around that time.
Navarra could also be an interesting question. OTL, after Charles IV's death, the crown of Navarra was given to Jeanne de Navarre, Louis X's daughter, because Philippe de Valois had technically no rights to the the Navarese crown. The scenario wouldn't necessarilly happen if Charles IV has a surviving son: Navarra could thus stay in personnal union with France as they'd have the same King.
Also mentionning Navarra because of Charles the Bad, who could still be a threat for his cousin. He wouldn't necessarilly be as strong as he was OTL and he wouldn' be able to take advantage of the Hundred Years' War... But Charles the Bad doesn't really strike me as someone who wouldn't try something: he seems to ambitious and proud for that. Not to mention that he still has some solid claims to a lot of inheritance... and is technically a rival for the French crown if you consider Salic Law invalid. Probably would go as good for him as it did OTL though, if not worse given that the Hundred Years War would basically not be a thing, even if England and France are likely going to go to war over Guyenne.