 He was the second son of Bolesław Januszowic (in turn second son of Janusz I of Warsaw) and Anna Feodorovna of Ratnie, a Lithuanian princess (granddaughter of Algirdas).The successive deaths of his older uncle, father and brother between 1422–1427 left Bolesław IV as the only surviving male heir of Janusz I, who in order to protect his rights, in 1428 forced his local nobility to paid homage to the seven-year old prince as his future ruler; thanks to this, when the Duke died one year later (8 December 1429), Bolesław IV could assumed the power without much difficulties; however, because he was a minor at that moment, his mother Anna took the regency on his behalf until 1436, when he attained his majority and began his personal rule.In foreign policy, Bolesław IV initially tried to stay neutral in the conflicts between the Teutonic Order and the Kingdom of Poland (and his ally the Grand Duchy of Lithuania), but soon he was forced to choose a side in 1431, at the outbreak of the Polish–Teutonic War.