Gaston IV was the sovereign Viscount of Béarn and the Count of Foix and Bigorre in France from 1436 to 1472. He also held the viscounties of Marsan, Castelbon, Nébouzan, Villemeur and Lautrec and was, by virtue of the county of Foix, co-prince of Andorra. From 1447 he was also Viscount of Narbonne. Through his marriage to Eleanor, heiress of the Kingdom of Navarre, he also held the title of Prince of Navarre.

The viscounts of Béarn were the rulers of the viscounty of Béarn, located in the Pyrenees mountains and in the plain at their feet, in southwest France. Along with the three Basque provinces of Soule, Lower Navarre, and Labourd, as well as small parts of Gascony, it forms the current département of Pyrénées-Atlantiques (64).