Four-part harmony is music written for four voices, or for some other musical medium—four musical instruments or a single keyboard instrument, for example—for which the various musical parts can give a different note for each chord of the music.
The four main voices are typically labelled as soprano (or treble and countertenor), alto (contralto, countertenor or mezzo), tenor, and bass. Because the human voice has a limited range, different voice types are usually not able to sing pitches that lie outside of their specific range.