Pope John IV, a strong defender of orthodoxy, stood against Monothelitism, aided his homeland of Dalmatia, and reinforced key Church traditions during his papacy from AD640 to AD642.

Pope John IV served as the Supreme Pontiff of the Holy Catholic Church from December 24, AD640, to October 12, AD 642.

He was born in Dalmatia (present-day Croatia), then a part of the Roman Empire, to a lawyer named Venantius. Although his father was a legal advisor to the Exarch, the emperor's representative in Rome, Pope John IV refused to accept the royal position in favor of the Monothelite heresy. He also took a strong stand against Monothelitism. Within a month of his consecration ie, in January AD 641, Pope John IV convened a synod in Rome and condemned Monothelitism as a heresy.

When the new Patriarch of Constantinople, Pyrrhus I, appealed to Pope John IV to recognize Monothelitism as an official doctrine—citing Pope Honorius's silent acceptance of the Ecthesis of Emperor Heraclius, which was a clear expression of Monothelitism—the Pope rejected the patriarch's request. Moreover, expressing his dissatisfaction and displeasure with Patriarch Pyrrhus’ attempts to associate his predecessor's name with heretical teachings, Pope John IV asked the new Roman Emperor, Constantine III, to stop displaying copies of the Ecthesis in public places in Constantinople.

The invasion of his native Dalmatia by Slavonic peoples turned away the Pope's attention. Pope John IV sent Abbot Martin to Dalmatia to relieve the suffering of his people and to secure the release of the Slavonic captives. Later, he contributed to the construction of a chapel near the baptismal font in the Lateran Basilica to house the relics of the martyrs of Dalmatia that Martin had brought to Rome.

John IV followed a different set of customs and traditions regarding the election of a pope. He had to wait about five months for the emperor to officially approve his election. On that occasion, the Roman Church leadership issued strict instructions to the bishops and abbots of Ireland, rebuking the custom of celebrating Easter based on the Jewish Passover—unlike other Church communities—and demanding that they change this custom to celebrate Easter together with other Christian communities. They also warned against the heresy of Pelagianism. However, the document was signed by Pope John IV as the second person. Although he was elected head of the Church, the first to sign the official document for the Church in Ireland were Archpriest Hilary and Chief Secretary John, as representatives of the Holy See.

For much of Church history, a papal election alone did not confer full legitimacy; the pope also had to be consecrated as Bishop of Rome. This applied to Pope John IV, whose election was not valid until his episcopal consecration. This principle remains in modern canon law (Canon 332.1), which states that if the elected pope is not a bishop, his election is not valid until he is consecrated as a Bishop.