William IV (William Alexander; French: Guillaume Alexandre) reigned as the Grand Duke of Luxembourg from 17 November 1905 until his death. He succeeded his father, Adolphe.
William 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 his heirs 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 as heir presumptive to the grand ducal throne. She became Luxembourg's first reigning grand duchess upon her father's death in 1912, and upon her own abdication in 1919, was succeeded by her younger sister Charlotte. Charlotte's descendants reign until the present day.
He was the last monarch of Luxembourg to die whilst still on the throne.