(Fourth Movement) B-flat major, 4/4 time / 3/4 time

The introduction, which begins with the repetition of F a semitone lower than the previous movement, seems to be exploring tonality throughout. It passes through G-flat major, B major, G-sharp minor, and other keys, finally reaching A major. Amidst the repeated A notes, the bass proceeds to F, functioning as the dominant of the main section in B-flat major.

The main section is marked "Fuga tre voci, con alcune licenze" (Three-voice fugue, with some liberties). The fugue subject is highly distinctive, featuring a leap of a tenth, a trill, and a sequential progression of sixteenth notes with chromatic alterations. The countersubject has a rhythmic characteristic with an accent placed on the second beat, and the subsequent free section is in a complementary rhythmic relationship with the countersubject.

The next section modulates to A-flat major, where the fugue unfolds with the latter half of the fugue subject, the latter half of the countersubject, and a new countersubject progressing sequentially in thirds/sixths. Subsequently, it transitions to G-flat major, E-flat minor, D-flat major, A-flat major, and other keys, developing a fugato that utilizes musical ideas spun out from the theme, and even trills, as motives.

Entering a new section in B minor, the fugue develops with the retrograde form of the subject. After making a half cadence on the dominant of D minor, a new cantabile-style fugue emerges in D major. It then transitions to B-flat major, where the original fugue subject re-enters. Along with this, variations of previously appeared themes and countersubjects emerge, further recalling motives with a trill in the bass as an organ point. The piece concludes with a portion of the fugue subject and trills in octaves.