count of Boulogne (from 1150) and eldest son of King Stephen of England and his wife Matilda, daughter and heiress of the previous count of Boulogne (Eustace III).

Eustace IV did homage for Normandy (1137) to Louis VII, king of France, whose sister Constance he later married (1140), and he was several times used by the king as an opponent to the claims on the Norman duchy made by the counts of Anjou.

At a council held in London on April 6, 1152, King Stephen induced some English barons to pay homage to his son Eustace as their future king, but Theobald, archbishop of Canterbury, on the command of Pope Eugenius III, refused to crown him. Eustace, whom contemporaries respected only as a soldier, was killed while plundering the abbey of Bury St. Edmunds in Suffolk. His death made possible a settlement of the civil war between Stephen and the empress Matilda. Stephen designated as his heir Matilda's son Henry of Anjou, afterward Henry II of England.