The fourth has a dual nature: acoustically consonant, yet in classical harmony often treated as a dissonance that must resolve to a third.
The ascending fourth is a “fanfare” interval: most military signals (reveille, charge) are built on it. Famous examples include the opening of Wagner's Wedding March and the first notes of the US national anthem, The Star-Spangled Banner. In jazz, quartal harmony (chords built in fourths) is a signature sound of McCoy Tyner and Herbie Hancock.
The perfect fourth is the inversion of the perfect fifth (together they make an octave), and like the fifth it is stable and open in sound. In the classical contrapuntal tradition a fourth above the bass counted as a dissonance, while between upper voices it sounded as a consonance — a paradox that shaped the rules of voice leading for centuries. The ascending fourth is easy to recognise from the opening of many anthems and fanfares.
