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.
a four part harmony refers to music that requires four musical instruments.
music in four-part harmony can utilize four musical instruments but can also be done with a single keyboard.
( z1 refer :ARG0 ( z2 harmony :mod ( z3 part :quant 4 ) ) :ARG1 ( z4 music :ARG0-of ( z5 require :ARG1 ( z6 instrument :quant 4 :mod ( z7 musical ) ) ) ) )