William Cavendish, 4th Duke of Devonshire, KG, PC (8 May 1720 – 2 October 1764), styled Lord Cavendish before 1729, and Marquess of Hartington between 1729 and 1755, was a British Whig statesman and nobleman who was briefly nominal 5th Prime Minister of Great Britain. He was the first son of William Cavendish, 3rd Duke of Devonshire and his wife, Catherine Hoskins. He is also a great-great-great-great-great-grandfather of King Charles III through the king's maternal great-grandmother.