Constantine IV (Greek: Κωνσταντῖνος Δ', Kōnstantinos IV, Latin: Flavius Constantinus Augustus) (c. 652 – 14 September 685), sometimes incorrectly called Pogonatos, "the Bearded", out of confusion with his father, was Byzantine Emperor from 668 to 685. His reign saw the first serious check to nearly 50 years of uninterrupted Islamic expansion, while his calling of the Sixth Ecumenical Council saw the end of the monothelitism controversy in the Byzantine Empire.
The eldest son of Constans II, Constantine IV had been named a co-emperor with his father in 654. He had been given the responsibility of managing the affairs at Constantinople during his father’s extended absence in Italy and became senior Emperor when Constans was assassinated in 668. His mother was Fausta, daughter of patrician Valentinus.
The first task before the new Emperor was the suppression of the military revolt in Sicily under Mezezius which had led to his father's death. Within seven months of his accession, Constantine IV had dealt with the insurgency with the support of Pope Vitalian. But this success was overshadowed by troubles in the east.