The term "four-part harmony" refers to music written for four voices or for some other musical medium—four musical instruments or a single keyboard instrument, for example—where the various musical parts can give a different note for each chord of the music.
four-part harmony is a special form of music
( z1 harmony :mod ( z2 form :ARG1-of ( z3 special ) :mod ( z4 music ) ) :mod ( z5 part :quant 4 ) )