Alexios IV Angelos or Alexius IV Angelus (Greek: Αλέξιος Δ' Άγγελος) (c. 1182 – February 8, 1204) was Byzantine Emperor from August 1203 to January 1204. He was the son of emperor Isaac II Angelus and his first wife Irene. His paternal uncle was Emperor Alexius III Angelus.
While there he met with Marquis Boniface of Montferrat, Philip's cousin, who had been chosen to lead the Fourth Crusade, but had temporarily left the Crusade during the siege of Zara to visit Philip. Boniface and Alexios discussed diverting the Crusade to Constantinople so that Alexios could be restored to his father's throne; in return, Alexios would give them 10,000 Byzantine soldiers to help fight in the Crusade, maintain 500 knights in the Holy Land, the service of the Byzantine navy (20 ships) in transporting the Crusader army to Egypt, as well as money to pay off the Crusaders' debt to the Republic of Venice with 200,000 silver marks. Additionally, he promised to bring the Greek Orthodox Church under the authority of the pope. Alexios accompanied Boniface back to the Crusader fleet, which had moved on to Corcyra, and the Venetians were in favour of the plan when they learned of it. In 1202 the fleet arrived at Constantinople. Alexios was paraded outside the walls, but the citizens were apathetic, as Alexios III, though a usurper and illegitimate in the eyes of the westerners, was an acceptable emperor for the Byzantine citizens.
Despite Alexios' grand promises, Isaac, the more experienced and practical of the two, knew that the Crusaders' debt could never be repaid from the imperial treasury. Alexios, however, had apparently not grasped how far the empire's financial resources had fallen during the previous fifty years. Alexios did manage to raise half the sum promised (100,000 silver marks), by appropriating treasures from the church and by confiscating the property of his enemies. He then attempted to defeat his uncle Alexios III, who remained in control of Thrace. The sack of some Thracian towns helped Alexios' situation a little, but meanwhile hostility between the restive Crusaders and the inhabitants of Constantinople was growing.
In December 1203 violence exploded between the Constantinopolitans and the Crusaders. Enraged mobs seized and brutally murdered any foreigner they could lay hands upon, and the Crusaders felt that Alexios had not fulfilled his promises to them. Alexios refused their demands, and is quoted as saying, "I will not do any more than I have done." While relations with the Crusaders were deteriorating, Alexios had become deeply unpopular with the Greek citizenry, and with his own father. Blinded and nearly powerless, Isaac II resented having to share the throne with his son; he spread rumors of Alexios' supposed sexual perversity, alleging he kept company with "depraved men". The chronicler Nicetas Choniates dismissed Alexios as "childish" and criticized his familiarity with the Crusaders and his lavish lifestyle. At the beginning of January 1204, Alexios IV retaliates against the Crusaders by setting fire to 17 ships and sending it against the Venetian fleet, but the attempt fails.