Leo IV was a 9th-century pope and bishop of Rome who led the Papal States from 847 until his death in 855. Born in Rome, he was educated at the monastery of St. Martin and served as a cardinal-priest before his unanimous election to the papacy. He is primarily remembered for his defensive efforts to protect the city from Saracen raids, including the construction of the Leonine Wall and the successful naval victory at the Battle of Ostia.