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 who was briefly titular Prime Minister of Great Britain.

He was elected MP for Derbyshire in 1741 and 1747, but left the House of Commons for the Lords in 1751 by a writ of acceleration as Baron Cavendish and was sworn of the Privy Council. He was Lord Lieutenant of Ireland from 2 April 1755 until 3 January 1757, when his successor, the 4th Duke of Bedford, entered the office. After inheriting his father's peerage titles (chiefly the Dukedom of Devonshire), Devonshire was given the Garter and appointed First Lord of the Treasury (most historians consider him Prime Minister during this service) in November 1756, and he served as First Lord until May 1757 in an administration effectively run by William Pitt the Elder. In 1762, he was sworn of the Privy Council of Ireland.

He married Lady Charlotte Elizabeth Boyle, Baroness Clifford (1731–1754), the daughter and heiress of the 3rd Earl of Burlington of the 1st creation, a famous architect and art collector. Through her, the Devonshires inherited Chiswick House and Burlington House in London; Bolton Abbey and Londesborough Hall in Yorkshire; and Lismore Castle in County Waterford, Ireland. The Duke employed Capability Brown to landscape the garden and park at Chatsworth House, his main residence. He hired James Paine to design the new stable block.

The Duke had four children:

• William Cavendish, 5th Duke of Devonshire (1748–1811)
• Lady Dorothy Cavendish (1750–1794). Married William Cavendish-Bentinck, 3rd Duke of Portland, who also became Prime Minister.
• Lord Richard Cavendish (1752–1781)
• George Augustus Henry Cavendish, created 1st Earl of Burlington of the 2nd creation (1754–1834). Lord Burlington's grandson, the 2nd Earl of Burlington, would later inherit the Devonshire dukedom as 7th Duke of Devonshire.