tetralogy

n. [ Gr. &unr_;; te`tra- (see Tetra-) + &unr_; a speech, discourse: cf. F. tétralogie. ] (Gr. Drama) A group or series of four dramatic pieces, three tragedies and one satyric, or comic, piece (or sometimes four tragedies), represented consequently on the Attic stage at the Dionysiac festival. [ 1913 Webster ]

☞ A group or series of three tragedies, exhibited together without a fourth piese, was called a trilogy. [ 1913 Webster ]