Siemowit IV, also known as Siemowit IV the Younger, was a Polish prince, member of the Masovian branch of the House of Piast and from 1373 or 1374 Duke of Rawa, and after the division of the paternal inheritance between him and his brother in 1381, ruler over Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna, after 1386 hereditary Polish vassal, after 1388 ruler over Belz. During 1382–1401 he lost Wizna and during 1384–1399 and 1407–1411 he lost Zawkrze, during 1384–1399 he lost Płońsk, which was taken by the Teutonic Order.

Duke of Masovia was a title borne by the sons and descendants of the Polish Duke Bolesław III Wrymouth. In accordance with the last will and testament of Bolesław, upon his death his lands were divided into four to five hereditary provinces distributed among his sons, and a royal province of Kraków reserved for the eldest, who was to be High Duke of all Poland. This was known as the fragmentation of Poland. Subsequent developments lead to further splintering of the duchies.