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 and be composed part by part
( z1 and :op ( z2 harmony :mod ( z3 part :quant 4 ) ) :op ( z4 compose :ARG1 z2 :manner ( z5 part-by-part ) ) )