William IV (22 April 1852 – 25 February 1912) reigned as the sovereign Grand Duke of Luxembourg from 17 November 1905 until his death. He succeeded his father, Adolphe, Grand Duke of Luxembourg. He also held the titular title of Duke of Nassau.William IV was a Protestant, the religion of the House of Nassau. He married Princess Marie Anne of Portugal, believing that a Roman Catholic country ought to have a Roman Catholic monarch. Thus since William IV all other Grand Dukes have been Catholic.At the death of his uncle, Prince Nikolaus-Wilhelm in 1905, the only other legitimate male in the House of Nassau-Weilburg was William's cousin, Georg Nikolaus, Count of Merenberg, the product of a morganatic marriage. So in 1907, William declared the Counts of Merenberg non-dynastic, naming his own eldest daughter Marie-Adélaïde (1894–1924) as heiress-presumptive to the grand ducal throne. She became Luxembourg's first reigning female monarch upon her father's death in 1912, and upon her own abdication in 1919, was succeeded by her younger sister Charlotte (1896–1985). Charlotte's descendants have reigned until the present day.He was the last monarch of Luxembourg to die in office.