William IV may refer to:
- William IV of the United Kingdom (1765–1837)
- William IV, Duke of Aquitaine (937–994)
- William IV of Provence (died 1030)
- William, Margrave of Meissen (died 1062), also William IV of Weimar
- William IV, Count of Toulouse (c. 1040–1094)
- William IV, Marquess of Montferrat (1030–1100)
- William IV, Count of Nevers (1130–1168)
- William IV, Count of Ponthieu (1179–1221)
- William IV, Lord of Douglas (died 1333)
- William II, Count of Hainaut (1307–1345), also William IV of Holland
- William II, Duke of Bavaria (ruled 1404–1417), also William IV of Hainault
- William IV, Duke of Brunswick-Lüneburg (c. 1425–1503)
- William IV, Duke of Bavaria (1493–1550)
- William, Duke of Jülich-Cleves-Berg (1516–1592)
- William IV, Landgrave of Hesse-Kassel (or Hesse-Cassel) (1532–1592)
- William IV, Prince of Orange (1711–1751)
- William IV, Grand Duke of Luxembourg (1852–1912)
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