William Wentworth-Fitzwilliam, 4th Earl Fitzwilliam PC (1748 – 1833), styled Viscount Milton until 1756, was a British Whig politician. In 1782 he became one of the richest people in Britain when he inherited the estates of his uncle Charles Watson-Wentworth, 2nd Marquess of Rockingham.
Fitzwilliam accepted the Lord Lieutenancy on 10 August 1794 and wrote to Henry Grattan soon after:
"The chief object of my attempts will be, to purify, as far as circumstances and prudence will permit, the principles of government, in the hopes of thereby restoring to it that tone and spirit which so happily prevailed formerly, and so much to the dignity as well as the benefit of the country".
"I shall not do my duty if I do not distinctly state it as my opinion that not to grant cheerfully on the part of government all the Catholics wish will not only be exceedingly impolitick, but perhaps dangerous. ... If I receive no very peremptory directions to the contrary, I shall acquiesce with a good grace, in order to avoid the manifest ill effect of a doubt or the appearance of hesitation; for in my opinion even the appearance of hesitationmay be mischievous to a degree beyond all calculation."
Beresford and other Irish supporters of Ascendancy were alarmed at Fitzwilliam's policies and opposed him.
The official letter recalling Fitzwilliam was sent on 23 February 1795 with the King's wish that he should resume his old seat in the Cabinet (which Fitzwilliam rejected).
He continued to play a leading role in Whig politics until the 1820s.