On 11 August 1137, at the age of about 24, he was betrothed to the infant Petronilla of Aragon, aged one at the time. Petronilla's father, Ramiro II of Aragon, who sought Barcelona's aid against Alfonso VII of Castile, withdrew from public life on 13 November 1137, leaving his kingdom to Petronilla and Ramon Berenguer, the latter in effect becoming ruler of Aragon, although he was never king himself, instead commonly using the titles "Count of the Barcelonans and Prince of the Aragonians" (Comes Barcinonensis et Princeps Aragonensis), and occasionally those of "Marquis of Lleida and Tortosa" (after conquering these cities). He was the last Catalan ruler to use "Count" as his primary title; starting with his son Alfonso II of Aragon the counts of Barcelona styled themselves, in the first place, as kings of Aragon.
The treaty between Ramon Berenguer and his father-in-law, Ramiro II, stipulated that their descendants would rule jointly over both realms, and that even if Petronilla died before the marriage could be consummated, Berenguer's heirs would still inherit the Kingdom of Aragon. Both realms would preserve their laws, institutions and autonomy, remaining legally distinct but federated in a dynastic union under one ruling House.
In August 1150, when Petronilla was fourteen, the betrothal was ratified at a wedding ceremony held in the city of Lleida.[5] Petronilla consummated her marriage to Ramon Berenguer in the early part of 1151, when she reached the age of 15.
Ramon Berenguer IV died on 6 August 1162 in Borgo San Dalmazzo, Piedmont, Italy, leaving the title of Count of Barcelona to his eldest surviving son, Ramon Berenguer, who inherited the title of King of Aragon after the abdication of his mother Petronilla of Aragon two years later in 1164.