Opus Cuatro are an Argentine folk vocal quartet founded in La Plata in 1968. The group debuted on 10 July 1968 and become one of the most important vocal groups in Latin America, staying active since then without interruption. At the beginning of 2009, the quartet had carried out 7,100 performances in 450 cities worldwide, including 25 tours in Europe and nine in the United States.
The founder members were Alberto Hassan (first tenor), Antonio Bugallo (second tenor), Lino Bugallo (baritone) and Federico Galiana (bass). Bugallo left the group in 1972, and was replaced first by Hannibal Bresca and then Ruben Verna until Marcelo Balsells was incorporated in 1982 and has remained ever since.