Constantine IV (649-685) was Byzantine emperor from 668-685. He had been named a co-emperor with his father Constans II in 654, and became emperor when Constans was assassinated in 668.
The most immediate threat to the empire under his reign were the Arabs, who sent a fleet to attack Constantinople by sea in 674. While Constantine was diverted by this, the Slavs attacked Thessalonika.
Constantinople survived the Arab siege until 678, when the Byzantines employed Greek fire against the Arab fleet at the Battle of Syllaeum. This was one of the first times Greek fire was used in combat. The Arabs withdrew, and were almost simultaneously defeated on land in Anatolia.