Fantasia No. 4 in C minor, K. 475 is a composition for solo piano composed by Wolfgang Amadeus Mozart in Vienna on 20 May 1785.^[1] It was published as Opus 11, in December 1785, together with the Sonata in C minor, K. 457, the only one of Mozart's piano sonatas to be published together with a work of a different genre.^[2]

Mozart's Fantasia in C minor, K. 475

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Starting in the key of C minor, the piece is marked Adagio but then, after a section in D major, moves into an allegro section which goes from A minor to G minor, F major, and then F minor. It then moves into a fourth section in B♭ major marked Andantino and then moves to a più allegro section starting in G minor and modulating through many keys before the opening theme returns in the original key of C minor. Most of the music is written with no sharps or flats in the key signature and uses accidentals—only the fourth section, in B♭ major, is given a key signature.

The Austrian composer Ignaz von Seyfried combined this work with the Sonata in C minor, K. 457 and produced a four-movement arrangement for orchestra, the "Grande Fantaisie" in C minor.