John IV, Duke of Brittany

John IV of Montfort (in Breton Yann Moñforzh, in French Jean de Montfort) (1295 – 16 September 1345, Château d'Hennebont), was Duke of Brittany and 6th Earl of Richmond from 1341 to his death. He was the son of Arthur II, Duke of Brittany and his second wife, Yolande de Dreux.

Marriage and issue
In 1322 he succeeded his mother as count of Montfort, and in 1329, he married Joanna of Flanders at Chartres. They had two children:
 * John V, Duke of Brittany (1339–1399)
 * Joan of Brittany, Baroness of Drayton (1341 – aft. 20 October 1399), married before 21 October 1385 Ralph Basset, 4th Baron Basset de Drayton

Breton War of Succession
When his half-brother, John III, Duke of Brittany, died with no male heirs in 1341, John IV decided to contest the rights of his niece, Joanna of Penthièvre, and her husband, Charles of Blois, to the Duchy of Brittany. This led to the Breton War of Succession, which in turn became part of the Hundred Years' War. His patron in this quest was King Edward III of England.