Alexios IV Angelos or Alexius IV Angelus (Greek: ) (c. 1182 February 1204) was Byzantine Emperor from August 1203 to January 1204. He was the son of Emperor Isaac II Angelos and his first wife, an unknown Palaiologina, who became a nun with the name Irene. His paternal uncle was his predecessor Emperor Alexios III Angelos.



Isaac II Angelos (Greek: Ἰσαάκιος Κομνηνός Ἄγγελος, Isaakios Komnenos Angelos; September 1156 – January 1204) was Byzantine Emperor from 1185 to 1195, and again from 1203 to 1204.

His father Andronikos Doukas Angelos was a military leader in Asia Minor (c. 1122 – aft. 1185) who married Euphrosyne Kastamonitissa (c. 1125 – aft. 1195). Andronikos Doukas Angelos was the son of Constantine Angelos and Theodora Komnene (b. 15 January 1096/1097), the youngest daughter of Emperor Alexios I Komnenos and Irene Doukaina. Thus Isaac was a member of the extended imperial clan of the Komnenoi.