He was born in 1161, the son of Amalric I and his wife, Agnes of Courtenay. He spent his youth with his father, who had been obliged to divorce Agnes on grounds of consanguinity; when Amalric died, Baldwin was crowned King, at the age of 13. He was given a regent to govern for him: first Miles of Plancy; later Raymond of Tripoli, his father's cousin.
Most of the nobility did not expect Baldwin to live long, and they moved to gain influence over his heirs, Princess Sibylla and Princess Isabella.
Raymond had made a treaty, in Baldwin's name, with Saladin; on coming of age, at 15, Baldwin chose to disavow this, and began to raid the Muslim territories. He sought support from Byzantium for an attack on Egypt, and in 1177-8 defeated Saladin, in concert with Raynald of Chatillon and the Templars. However, in 1179, he suffered some reverses and lost the experienced Constable of the kingdom, and several other notable warriors.
Sibylla had been married, by the arrangements of Raymond of Tripoli, to William of Montferrat (who was a first cousin of both Louis VII of France and of the Emperor, Frederick I. By him she had a child, Baldwin who became heir to the throne, but he died at Ascalon, of illness. King Baldwin remarried her, to Guy of Lusignan (brother of his new Constable, Amalric); in 1182 the king, blind and unable to walk, also made Guy regent.
A year later, the two quarelled when Guy refused to join the king in fighting Saladin, who had beseiged the wedding festivities of Isabella and Humphrey of Toron. Guy was dismissed as regent, and Baldwin made his 5-year-old nephew his formal heir, excluding Sybilla from the succession, and making Raymond of Tripoli his guardian and putative regent, were the king to die and Baldwin the younger to succeed.
Baldwin V was crowned co-king in November 1183, and Baldwin IV moved to annul his sister's marriage to Guy. They declined to attend the annulment proceedings, and the dynastic struggle and an attempt to lift a siege of Kerak, a major castle of the kingdom, exhausted the King. His mother, who had been one of his staunch supporters, died toward the end of 1184, and Baldwin followed her the following spring.
Baldwin was an unusually strong king, handling both his leprosy and his politics valorously; his child-heir was able to succeed him, with Raymond as regent, and although the younger Baldwin was fated to reign for but a year his death was natural, rather than a political act as part of a coup.