High Quality Content by WIKIPEDIA articles! The perfect fourth ( About this sound Play (help·info)) is a musical interval which spans four scale degrees. It consists of the note and the note five semitones above it on the musical scale. For example, the interval between a C and the next F above it is a perfect fourth; similarly the interval between a G and the next C above it, between an F and the B flat above it, and so on. The perfect fourth may be derived from the harmonic series as the interval between the third and fourth harmonics.