Ermengol (or Armengol) IV (1056–1092), called el de Gerb or Gerp, was the Count of Urgell from 1066 to his death. He was the son of Ermengol III and Clemencia, daughter of Bernard II of Bigorre.
Ermengol inherited Urgell when he was only ten years old and ruled under the tutelage of the countess dowager, Sancha, third wife of his father, until he was twelve. During this brief minority, the nobility took the opportunity to plunder and occupy the comital demesne. It was not until 1075 that Ermengol was in control of his county and his nobles.
Ermengol was an active count. During his reign, Urgell profited economically by receiving exiles from Lleida and Fraga. In 1076, having brought the nobles to submission, he began a Reconquista of his own, taking the basin of the river Sió with the villages of Agramunt and Almenar that year and Linyola and Belcaire in 1091. He conquered Calassanç and built a castle at Gerb, Spain, where he died, in an effort to pave the way to the recapture of Balaguer, which occurred during the reign of his son, Ermengol V, in 1102.
Ermengol was a firm supporter of the contemporary Gregorian reform of the Church, which he introduced to Urgel.
In 1077, Ermengol married Lucy, daughter of Bernard I of La Marche. With her, he had his son and heir, the aforementioned Ermengol. In 1079, he remarried to Adelaide Bertrand, daughter of William Bertrand of Provence. She bore him one son, William, who inherited Forcalquier, and a daughter who died young.