Arthur Wills Blundell Sandys Trumbull Windsor Hill, 4th Marquess of Downshire KP (6 August 1812 – 6 August 1868) was an Irish peer, styled Earl of Hillsborough until 1845.
The eldest son of Arthur Hill, 3rd Marquess of Downshire, Hillsborough was educated at Eton and Christ Church, Oxford, where he matriculated in 1830. He was commissioned an ensign in the Royal South Down Militia, of which his father was colonel, on 4 June, and was commissioned lieutenant-colonel in the same on 10 September.
He was appointed Sheriff of County Down for 1834. From 1836 until 1845, he represented Down in Parliament, and was a justice of the peace for the county as well.
He became Marquess of Downshire on 12 April 1845 on the death of his father, and was appointed to his father's Militia colonelcy on 30 July. His English residence was Easthampstead Park in Berkshire, and he was appointed a deputy lieutenant of that county in 1852, and a Knight of the Order of St Patrick on 24 May 1859. He had Easthampstead Church rebuilt in 1867.