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 pay 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.

Michael was briefly married with Bolesław IV's sister Euphemia in 1435–1436, and after his father's murder he was forced to seek refuge at the court of his brother-in-law in 1440.

At this point the Polish decided to aid Bolesław IV and announced that they soon send their troops; however, at the same time, they sent representatives to negotiate an agreement with the Lithuanians.

The closest male relative and heir apparent was the Grand Duke Casimir of Lithuania, but the local magnates refused to renew the union between both countries.

Previously, on 30 March the Crown Council was reunited and considered two main candidates: Frederick II, Elector of Brandenburg (supported by the Polish Episcopate) and Bolesław IV.

Despite this, Bolesław IV decided to support the rights of Casimir, in order to neutralize the danger that suppose the election of Frederick II of Brandenburg as King.

In domestic politics Bolesław IV continued the line of his grandfather Janusz I, taking care of the local legislation under the Kulm law.