The Chronicon Dubnicense names "Belam, Colomannum Andream et beatam Elyzabeth" as the children of "Andreas filius Bele" and his wife "domina Gerdrudis de Alemania". The Gesta Hungarorum names "Bela filius eius" when recording that he succeeded his father. Béla was governor of Hungary's eastern provinces as rex iunior, and represented his father at the mass baptism of the Kuman people which took place in 1227 in Moldavia on the orders of Bortz Khan.
He succeeded his father in 1235 as BÉLA IV King of Hungary. He did not go to Croatia for a second coronation, breaking the custom which had lasted 130 years. In response to the Pope's call in 1234 for a crusade against 'heretics' in Bosnia, Hungary occupied large parts of Bosnian territory between 1235 and 1241, but was obliged to withdraw in the face of the threat from the Mongols. Kuthen [Kötöny] Khan of the Kumans and his followers sought refuge in Hungary after the battle of the Volga in 1239, which gave the Mongols the pretext to attack Hungary. Their presence caused internal resentment in Hungary which culminated in the Khan's murder. Batu Khan's forces passed the Carpathian mountains into Hungary in Feb 1241 and defeated Béla IV at Muhi on the river Sajó 11 Apr 1241. King Béla fled to the castle of Knin in Dalmatia after this defeat, then to Trogir/Trau and finally to the island of Ciovo, but the Mongols withdrew in 1242 after hearing news of the death of Great Khan Ogedei 11 Dec 1241 at Karakoram. Relations with Bulgaria improved in 1240, possibly because of the threat posed to both states by the Mongols. Hungary was also attacked on its western borders by Friedrich Duke of Austria, although for a short time King Béla captured Styria in reprisal. In order to protect his eastern frontier, King Béla contracted with the Knights of the Order of St John of Jerusalem [Knights Hospitalers] 2 Jun 1247, granting them the Szörénység as far as south-eastern Transylvania in return for the promise of military support. They remained there until they relinquished the land in [1258/60]. King Béla allowed the Kumans to return to Hungary, settling them on uninhabited land on either side of the River Tisza, the agreement being sealed by the marriage of his son and heir to a Kuman princess in 1253. He was embroiled in a war with Bohemia, and suffered a defeat in 1260 despite reinforcements from Bulgaria supplied by his son-in-law Rostislav Mikhailovich Ban of Macva/Macsói, who was one of the claimants to the Bulgarian throne after Tsar Koloman II was deposed in 1258. Having made peace with Bohemia in Mar 1261, Hungary attacked Bulgaria, expelling Konstantin Tih (a rival claimant for the Bulgarian throne) from Vidin which he had captured from Rostislav's forces during the latter's temporary absence in Bohemia. Stefan Uroš I King of Serbia declared war on Hungary in 1268, plundered Macva/Macsói but was himself captured and held for ransom, the marriage between his son and Béla IV's granddaughter probably being agreed as part of the terms for the Serbian king's release.
The Annales Cracovienses Compilati record the death in 1271 of "Bela rex Hungarie pater ducisse Cracoviensis". The necrology of Oberaltaich records the death "VI Non Mai" of "Bela rex Ungarie". The Chronicon Dubnicense records the death "V Non May…in insula Budensi" in 1270 of "rex Bela" and his burial "Strigony in ecclesia fratrum minorum". The Gesta Hungarorum records that King Béla was buried "apud Fratres Minores Strigonii". 1
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