The term 'Slaughter of the Cardinals' refers to an event that occurred in 1572 during the French Wars of Religion. On August 24th, 1572, a group of French Catholics, known as the Huguenots, attacked and killed over two thousand French Catholics, known as the Cardinals, in Paris. The leader of the Huguenots was Admiral Gaspard de Coligny, and the event is also known as the St. Bartholomew's Day Massacre. The four letter answer to the crossword puzzle is ENOS, which stands for the French phrase 'Et Nous Osons, ' meaning 'And We Dare.'