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 succeeded his father Centulf V in 1090. During his rule, the borders of Béarn were established more definitively; he defeated the viscount of Dax, and took control of Orthez, Mixe , and Ostabaret by 1105. He also gained Montanérès through his marriage to Talèse, daughter of the count of Aibar and Javierrelatre and niece of Sancho I of Aragon. Though technically a vassal of the Duchy of Aquitaine, ruled at that time by William IX, Gaston effectively made Béarn an autonomous territory.

Before becoming viscount, Gaston had fought in the Reconquista in Spain, and he led a Béarnais contingent on crusade under Raymond IV of Toulouse in 1096. He was one of the lesser knights, but he carried his own standard and commanded his own men. 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 with his men, 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. He also allowed the abbey of St. Foy to establish new buildings in Morlàas. He also came into conflict with the church, however; he successfully defended his claims to the territories of the abbey of St. Vincent de Lucq and the monastery of St. Mont.

He died in 1131 and was succeeded by his young son Centulf VI , with Talèse acting as regent. Talèse wanted to unite Béarn and Aragon; the two were, at the time, roughly equal in power and influence, but Aragon instead united with Catalonia and Béarn began to decline. Gaston's descendants Gaston VI and Gaston VII participated in the Albigensian Crusade and the Seventh Crusade, respectively.

References

• Steven Runciman, A History of the Crusades, vol. 1: The First Crusade and the Foundation of the Kingdom of Jerusalem. Cambridge, 1951.
• Pierre Tucoo-Chala, La Vicomté de Béarn et le Problème de sa Souveraineté, des Origines à 1260. Bordeaux, 1961.