Upon succeeding his father he called a cort in Zaragoza for his coronation. He crowned himself, disappointing the Archbishop of Zaragoza and thus rejecting the surrender Peter II had made to the Papacy, in an otherwise traditional ceremony. According to his own later reports, this act caused him some "distress". He did, however, affirm the liberties and privileges of Aragon. Also while he was at Zaragoza an embassy from Castile had met him and asked that he promise to uphold the donations of land his father had made to his stepmother Eleanor, but he refused to give a clear answer as to the legitimacy of the donations.
After the festivities in Zaragoza, Peter began on his way to Valencia to receive coronation there. On route he stopped at Lleida to affirm the Usatges and Constitucions of Catalonia and receive the homage of his Catalan subjects. This offended Barcelona, at which the ceremony had usually been performed, and the citizens of that city complained to the king, who claimed that Lleida was on his way to Valencia. While in Valencia he decided on the case of his stepmother's inheritance, depriving her of income and outlawing her Castilian protector, Pedro de Ejérica. However, Pedro had enough supporters within Peter's domains that Peter was unable to maintain his position and in 1338, through papal mediation, Pedro was reconciled to the king and Eleanor received her land and jurisdictional rights. Peter was largely forced to capitulate by a new invasion from Morocco aimed at Castile and Valencia.
In 1338 he married Maria, second daughter of Philip III and Joan II of Navarre. In May 1339 he allied with Alfonso XI of Castile against Morocco, but his contribution of a fleet had no effect at the pivotal Battle of the Río Salado (October 1340).
Peter then opened a legal process against James, with the intent of dispossessing him of his kingdom. He alleged that the circulation of James' coinage in the Counties of Roussillon and Cerdagne to be an infringement on the royal right of monopoly of coinage. This was open to question, considering the ancient customs of Roussillon and Cerdagne, but Peter was prepared to move forward anyway. The interference of Pope Clement VI, however, granted James a hearing in Barcelona in front of papal delegates. Peter, for his part, spread rumours that James was seeking to capture him. James, fearing that Peter would stoop to invading Majorca and seizing it by force, returned to the island to prepare its defence. In February 1343 Peter declared James a contumacious vassal and his kingdom and lands forfeit.
The legal process being terminated, Peter went to war, on the advice that the islanders were burdened by taxes and would readily rise in his support. In May a fleet which had been blockading Algeciras landed at Majorca and quickly defeated James' army at the Battle of Santa Ponça. Peter received the submission of all the Balearics and confirmed the privileges of the islands as they had been under James I. Though James sued for peace and Pope Clement attempted to mediate it, Peter returned to Barcelona prepared to invade Roussillon and Cerdagne. After these were finally conquered in 1344 James surrendered on a safe conduct, only to find himself ignominiously reduced to the status of a petty lord. In March Peter had declared his realm incorporated into the Crown of Aragon in perpetuity and ceremoniously had himself crowned its king.
He found himself facing a rebellion among the nobles which would fail after he defeated the nobles in the Battle of Epila in 1348.
In 1356, he engaged with Peter I of Castile in what was called the "War of the Two Peters". It ended in 1375 with the Treaty of Almazán, without a winner due to the Black Death and several natural disasters.
He conquered Sicily in 1377 but the possession was given to his son Martin.
Throughout his reign, Peter IV had frequent conflicts with the inquisitor general of Aragon, Nicolau Aymerich.
In 1349, James invaded Majorca, but was soundly defeated by Peter's troops at the Battle of Llucmajor, in which he died. After James' death, Peter allowed James IV, his successor, to retain his royal title on purely formal terms until his death in 1375. After that date, Peter assumed the titular. Majorca remained one of the component crowns of the Crown of Aragon until the Nueva Planta decrees.
Toward the end of his reign (c. 1370) Peter ordered the compilation of the Chronicle of San Juan de la Peña to record the historical basis for the authority of the crown.
His third marriage was to Eleanor of Sicily (1325-1375), daughter of Peter II of Sicily. Four children were born from this marriage:
His last marriage, in 1377, was to Sibila of Fortià, who bore him a daughter:
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