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John IV of Montfort (in French Jean IV de Montfort) (1295 - September 16 1345), was duke of Brittany, from 1341 to his death. He was son of Duke Arthur II and Yolande de Dreux, countess of Montfort, his second wife. From his mother he became count of Montfort.
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. This lead to the Breton War of Succession an event of the Hundred Years War. His patron on this quest was king Edward III of England. John IV died without accomplishing his objective of becoming sole ruler of Brittany, but his son John continued the fight and eventually won the Succession War.
| Preceded by: John III | Duke of Brittany disputed with Joanna and Charles | Succeeded by: John V |