Leo IV, Saint, d. 855, pope (847-55), a Roman; successor of Sergius II. He had seen the Saracen attack on Rome (846), and to prevent its recurrence he fortified the city and its suburbs. He built a wall around the Vatican, established and fortified the part of Rome still called the Leonine City, and rebuilt churches. He crowned Louis II Holy Roman emperor. Leo was succeeded by Benedict III. Feast: July 17.

Leo IV (Leo the Khazar), d. 780, Byzantine emperor (775-80), son and successor of Constantine V. He owed his nickname to his mother, a Khazar princess. Leo tempered the iconoclastic excesses of his father's reign. On his death his Athenian wife, Irene, became regent for their son, Constantine VI.