Siemowit IV (Ziemowit IV), also known as Siemowit IV the Younger (pl: Siemowit IV Młodszy; ca. 1353/1356[1] – 21 January 1426[2]), 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.
He was the second son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava.
Already during his father's lifetime, Siemowit IV received his own district, Rawa Mazowiecka (ca. 1373/74), and as a result of the partition of Masovia between him and his older brother Janusz I after the death of their father on 16 June 1381, Siemowit IV finally obtain the totality of his domains: Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna.