Eustace IV (c. 1130 - August 1153) was a Count of Boulogne and the son and heir of King Stephen of England. He became the Heir Apparent to his father's lands by the death of an elder brother before 1135, and inherited Boulogne through his mother, Matilda of Boulogne.

In 1137, he did homage for Normandy to Louis VII of France, whose sister, Constance, he subsequently married in 1040 (as a widow she remarried to Count Raymond V of Toulouse). Eustace was knighted in 1147, at which date he was probably from sixteen to eighteen years of age. In 1151 he joined Louis in an abortive raid upon Normandy, which had accepted the title of the Empress Maud, and was now defended by her husband, Geoffrey of Anjou.

At a council held in London on April 6, 1152, Stephen induced a small number of barons to do homage to Eustace as their future king; but the primate, Theobald, and the other bishops declined to perform the coronation ceremony on the grounds that the Roman curia had declared against the claim of Eustace.

Eustace died suddenly the next year, in early August 1153 struck down (so it was said) by the wrath of God while plundering church lands near Bury St. Edmunds. The death of Eustace was hailed with general satisfaction as opening the possibility of a peaceful settlement between Stephen and his rival, the young Henry of Anjou. According to William of Newburgh, King Stephen was "grieved beyond measure by the death of the son who he hoped would succeed him; he pursued warlike preparations less vigorously, and listened more patiently than usual to the voices of those urging peace."

The Peterborough Chronicle, not content with voicing this sentiment, gives Eustace a bad character. "He was an evil man and did more harm than good wherever he went; he spoiled the lands and laid thereon heavy taxes." He had used threats against the recalcitrant bishops, and in the war against the Angevin party had demanded contributions from religious houses; these facts perhaps suffice to account for the verdict of the chronicler.