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 often played in an orchestra 
( z1 play :ARG1 ( z2 harmony :mod ( z3 part :quant 4 ) ) :frequency ( z4 often ) :location ( z5 orchestra ) )