William Cavendish, 4th Duke of Devonshire (c. 1720 - 2 October 1764) was a British statesman who eventually became 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 of Hardwick. He was Lord Lieutenant of Ireland from 2 April 1755 until 3 January 1757, when his successor John Russell, 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.
He married Lady Charlotte Boyle (1731 - 1754), the daughter and heiress of Richard Boyle, 4th Earl of Cork and 3rd Earl of Burlington, 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.
|- style="text-align: center;"
| width="30%" |Preceded by:
The Duke of Dorset
| width="40%" style="text-align: center;" |Lord Lieutenant of Ireland
1755–1757
| width="30%" |Succeeded by:
The Duke of Bedford
|- style="text-align: center;"
| width="30%" |Preceded by:
The Duke of Grafton
| width="40%" style="text-align: center;" |Lord Chamberlain
1757–1762
| width="30%" |Succeeded by:
The Duke of Marlborough
