William O'Brien, 4th Earl of Inchiquin

William McWilliam O'Brien, 4th Earl of Inchiquin, KB, PC(I) (1700 – 18 July 1777) was an Irish peer and politician.

O'Brien was the eldest son of William O'Brien, 3rd Earl of Inchiquin and his wife, Mary (née Villiers), sister of the 1st Earl of Jersey, and inherited his father's titles in 1719. He founded the predecessor of the present day Royal Cork Yacht Club, the Water Club of the Cork Harbour, in 1720.

Inchiquin was Whig Member of Parliament for Windsor from 1722 to 1727, for Tamworth from 1727 to 1734, for Camelford from 1741 to 1747 and for Aylesbury from 1747 to 1754. In 1725, he had been appointed a Knight Companion of the Order of the Bath. From 1740 to 1741, he was Grand Master of the Premier Grand Lodge of England, Governor of County Clare from 1741 to 1777 and Custos Rotulorum of County Clare from 1762 to 1767. From 1744 to 1751, he was a Lord of the Bedchamber to Frederick, Prince of Wales and admitted to the Privy Council of Ireland in 1753. On his death in 1777, without surviving male heirs, his titles passed to his nephew, Murrough O'Brien, who was later created Marquess of Thomond.