I'm looking for a Rainbow Rocks story where there was a fourth younger siren.

Oh, I think I know that story

6699411
Is this it?

The naming pattern really does suggest a missing 4th siren,
Aria and Sonata are opposites, since an aria is a solo vocal performance and a sonata is a group instrumental performance.
Adagio is a tempo term meaning 'slow', so the missing Siren would be Allegro, for fast.

A contrasting personality for Adagio and the others to play off, since Sonata and Aria are always playing off each other and Adagio is basically forced to referee.

Ok, all headcanon, and maybe not relevant.

6699447
No, the fourth Siren's name is Melody, but thanks anyways.

6699871
I really like this idea. It's a very nice bit of reasoning