Agnetha, Björn, Benny and Anni-Frid as a group (4)
I believe the answer is:
abba
(Other definitions for abba that I've seen before include "rhymes in Petrarchan sonnet" , "group of four stars?" , "Bishop" , "Coptic bishop's title; Swedish pop group" , "Palindromic Swedish pop group who sang ` `Waterloo' '" .)