Siemowit IV (Ziemowit IV), also known as Siemowit IV the Younger (pl: Siemowit IV Młodszy; ca. 1353/1356 – 21 January 1426), was a Polish prince member of the House of Piast from the Masovian branch, from 1373/74 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, since 1386 hereditary Polish vassal, since 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, 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. Role in the Greater Polish Civil War Siemowit IV was opposed to his older brother Janusz I's attempts to obtain the Polish crown. One year after the acquisition of his own domains, King Louis of Poland and Hungary died (10 September 1382), and with this emerged the opportunity to enact his claim to the crown, supported by the Greater Poland and Kujawy nobility (centered around the powerful Bartosz Wezenborg). However, the late King had made arrangements among the Lesser Poland nobility who guaranteed their support to his eldest daughter and heiress Mary and her husband Sigismund of Luxembourg. Without waiting for a favorable settlement, in January 1383, Siemowit IV marched to Greater Poland at the head of his troops, marked the beginning of the Greater Poland Civil War. Unexpectedly, in Buda the Dowager Queen Elizabeth of Bosnia decided to change the decision of her husband and accepted the reign of her youngest daughter Jadwiga over Poland instead of Mary and Sigismund, who remained rulers of Hungary. This decision caused that several supporters of Siemowit IV to hope that he could marry the young Queen (despite the fact that she was already betrothed to William of Habsburg) and in this way, both factions could reconcile and in addition this union with the old Piast dynasty could further legitimize the Angevin rule. The first step