The term 'SOS Pop Quartet' refers to the Swedish pop group ABBA, which was formed in 1972 and consisted of four members: Agnetha Fältskog, Björn Ulvaeus, Benny Andersson, and Anni-Frid Lyngstad. The group achieved international success with their catchy pop songs and became one of the most successful musical acts of the 1970s. They are best known for their hit songs 'Dancing Queen, ' 'Mamma Mia, ' and 'SOS.'