Byzantine emperor from 1203 to 1204. Alexius was the son of Emperor Isaac II (Isaac II Angelus). He regained control of his rights to the Byzantine throne with the help of the Fourth Crusade (Crusades) but was deposed soon after by a palace coup.

Imprisoned in 1195 with his father (who had been blinded) by Alexius III (Alexius III Angelus), he escaped in 1201 and joined his sister and her husband, Philip of Swabia (Philip), in Germany. Promising funds, supplies, and troops to conquer Egypt, the maintenance of 500 knights in the Holy Land, and submission of the Byzantine church to Rome, he convinced Philip of Swabia, Crusade leader Boniface of Montferrat, and their Venetian allies to divert the Fourth Crusade to Constantinople in order to reinstate him and his father as coemperors; this plan was accomplished in 1203. The new coemperors, however, were unable to pay their debts to the West or to unite the two churches. Heavy taxation, as well as wanton behaviour on the part of the Crusaders in Constantinople, caused a national revolt led by Alexius Ducas Mourtzouphlus, the son-in-law of Alexius III. Alexius Ducas, who was proclaimed emperor in January 1204 as Alexius V (Alexius V Ducas Mourtzouphlus), had Alexius IV strangled. Isaac died in prison a few days later.