Quartet's initial scheme for 2, perhaps (4)

I believe the answer is:

abba

'quartet's' is the definition.
(ABBA, pop band quartet)

I cannot understand how the rest of the clue works.

(Other definitions for abba that I've seen before include "Enclosed rhyme" , "famous pop group" , "striped coat?" , "1970s Swedish pop group" , "They used to record" .)

I've seen this clue in The Guardian.

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