John IV of Montfort was a French nobleman born in 1330, known for his role as the Duke of Brittany. He was a member of the House of Montfort, which played a significant part in the political landscape of medieval France. His reign was marked by conflicts over the duchy, particularly with the House of Blois, leading to the Brittany War. John IV became Duke in 1364 after a series of battles and negotiations. He was recognized for his efforts to stabilize the region and strengthen its autonomy. His leadership helped to solidify the Montfort family's influence in Brittany until his death in 1399.