Prelude No. 4 is a fast and lively piece in 3/8 and is written in regular three-part song form. The texture of the piece is primarily eighth notes. All phrases are irregular, the piece is primarily a modal mixture of the parallel keys of B major and B minor, the melody is similar in character to a Spanish gypsy song, and the harmonies include Neapolitan and extended chords. These developments exemplify the combination of folk, impressionist, and modern influences that is a constant throughout the work. The A section (mm 1-15) utilizes a B pedal tone from mm 1-7 and the harmonic accompaniment alternates between B major and B minor throughout the A section. This is followed by a transition that consists of an unaccompanied scale passage from mm 16-21 that leads into the B section. The B section features a stark change of texture to a largely unaccompanied melody from mm 25-28 that possesses characteristics that are similar to melodies sung in the style of Cante Jondo, a vocal and poetic style of Andalusian flamenco music that is known for setting melodies of a small pitch range to lyrics that typically convey deeply emotional subject matter. Like most Cante Jondo music, the melody features a narrow range and is primarily unaccompanied. Shortly thereafter, the key changes to G# minor, the relative minor of B major, and there is an extended half cadence from mm 33-36 leading to a varied repetition of the melody that first appeared in mm 25-28. There is also an F# pedal tone and impressionistic chord voicings from mm 41-49. The first phrase of the A section (A’) comes back in slightly modified form from mm 53-58, which is followed by a ten measure Coda that alternates between B major and its neapolitan chord, C major, ending on an imperfect authentic cadence in B major.
For more on Flamenco and Spanish gypsy music, check out this dissertation: https://aquila.usm.edu/cgi/viewcontent.cgi?article=1846&context=dissertations (go to pages 5-6 to learn more about Cante Jondo music)