Pope John IV (Latin: Ioannes IV) reigned from 24 December 640 to his death in 642. His election followed a four-month sede vacante.
Troubles in his native land caused by invasions of Slavs directed John's attention there. To alleviate the distress of the inhabitants, John sent the abbot Martin into Dalmatia and Istria with large sums of money for the redemption of captives. As the ruined churches could not be rebuilt, the relics of some of the more important Dalmatian saints were brought to Rome. John then erected an oratory in their honour. It was adorned by the pope with mosaics depicting John himself holding in his hands a model of his oratory. John endeavoured thereby to convert the Slavs in Dalmatia and Istria to Christianity. Emperor Constantine Porphyrogenitus claimed that Porga, duke of the Dalmatian Croats, who had been invited into Dalmatia by Heraclius, sent to Emperor Heraclius for Christian teachers. It is supposed that the Emperor to whom this message was sent was Emperor Heraclius himself, and that he sent it to Pope John IV.
John was buried in the Basilica of St. Peter.