Chess960: Puzzle no.14

Black has played a sloppy opening and seems to be dropping a pawn. It would be nice to quickly regain it, but how?

SP571 Black to play castling legal:
How can black regain the pawn?

(Solution bottom left corner)

Black can regain the pawn with:

4...Qg7! Inserting a mate threat, guarding the queen, preventing rook deflection off the f-file

5.c3 ... gxf5 and the pawn is saved