Gaston IV (died 1131), was viscount of Béarn from 1090 to 1131. He was called "le Croisé" ("the Crusader") due to his participation in the First Crusade.

Gaston had earlier fought in the Reconquista in Spain, and followed Raymond IV of Toulouse on crusade in 1096. He was one of the poorer knights, but he carried his own standard and had his own men under him. At the siege of Antioch he led one of the divisions in the final battle against Kerbogha. During the power struggle following the capture of Antioch, Gaston deserted Raymond for Godfrey of Bouillon and marched with him to Jerusalem. Gaston and Tancred were sent ahead of the main army to occupy Bethlehem, and during the siege of Jerusalem, Gaston was in charge of Godfrey's siege engines. On July 15, 1099, Gaston was the first crusader to enter the city.

Gaston's experience in the Reconquista taught him that Muslims could live under Christian rule, as mudejar. He preferred negotiation and dialogue to senseless massacre, and he and Tancred tried to protect some of the Muslims of Jerusalem by sheltering them in the Temple. However, these Muslims too were soon killed by other crusaders, enfuriating Gaston and Tancred. In August, Gaston led part of the centre line of the crusader army at the Battle of Ascalon. After the victory there, Gaston returned home, as did most of the other crusaders.

Gaston was a pious man, and upon his return to Béarn he oversaw the construction of many churches destined to shelter pilgrims on the route to Santiago de Compostela. His descendants Gaston VI and Gaston VII participated in the Albigensian Crusade and the Seventh Crusade, respectively.