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 can be compassed with any keyboard 
( z1 possible :ARG1 ( z2 compress :ARG1 ( z3 harmony :mod ( z4 part :quant 4 ) ) :instrument ( z5 keyboard :mod ( z6 any ) ) ) )