Upon his return to Hungary in May 1242, Béla found a country in ruins. Devastation was especially heavy in the plains east of the Danube where at least half of the villages were depopulated. The Mongols had destroyed most traditional centers of administration, which were defended by earth-and-timber walls. A severe famine followed in 1242 and 1243.

Preparation for a new Mongol invasion was the central concern of Béla's policy. In a letter of 1247 to Pope Innocent IV, Béla announced his plan to strengthen the Danube—the "river of confrontations"—with new forts. He abandoned the ancient royal prerogative to build and own castles, promoting the erection of nearly 100 new fortresses by the end of his reign. 

Béla attempted to increase the number of the soldiers and to improve their equipment. He made land grants in the forested regions and obliged the new landowners to equip heavily armoured cavalrymen to serve in the royal army. He even allowed the barons and prelates to employ armed noblemen, who had previously been directly subordinated to the sovereign, in their private retinue (banderium). 

To replace the loss of at least 15 percent of the population, Béla promoted colonization. He granted special liberties to the colonists, including personal freedom and favorable tax treatment. Germans, Moravians, Poles, Ruthenians and other "guests" arrived from neighboring countries and were settled in depopulated or sparsely populated regions. He also persuaded the Cumans, who had in 1241 left Hungary, to return and settle in the plains along the River Tisza. He even arranged the engagement of his firstborn son, Stephen, who was crowned king-junior in or before 1246, to Elisabeth, a daughter of a Cuman chieftain.