He played an important role in the Mad War, a series of conflicts in which aristocrats sought to resist the expansion and centralisation of power under the French monarch.

John incurred the enmity of King Louis XI of France when he supported the interests of Charles the Bold, duke of Burgundy.

Nephew of Duke Francis II of Brittany, John IV now took an active role in the affairs of the duchy, prompted by Maximilian.

He was placed under house arrest, but was released by King Charles VIII, in order to return to Brittany and prevent the marriage of Anne to Alain d'Albret.

He surrendered his own claim to the duchy for the large sum of 100,000 livres, and was reappointed as lieutenant general of Brittany, a position he held until his death.

His wife Philiberte de Luxembourg ordered an alabaster tomb from the sculptors Conrad Meyts and Giovanni Battista Mariotto.