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John was a native of Dalmatia, and the son of the scholasticus (advocate) Venantius.
As John's consecration (on November 24) followed very soon after his election, it is supposed that the papal elections were being confirmed by the Exarch of Ravenna rather than by the Emperor in Constantinople.
While still only pope-elect, John, with the other rulers of the Roman Church, wrote to the clergy of the North of Ireland to tell them of the mistakes they were making with regard to the time of keeping Easter, and exhorting them to be on their guard against the Pelagian heresy.
JohnIV of Montfort (in French Jean IV de Montfort) (1295 - September 16, 1345), was duke of Brittany, from 1341 to his death.
When his half-brother, duke John III, died with no male heirs in 1341, John decided to fight the rights of his niece Joanna of Dreux and her husband Charles of Blois to the duchy of Brittany.
JohnIV died without accomplishing his objective of becoming sole ruler of Brittany, but his wife Joanna of Flanders continued the fight in the name of their son John V who eventually won the Succession War.