Tetralogy
Tetralogy , noun
[Greek {not transcribed}; te`tra- (see Tetra-) + {not transcribed} a speech, discourse: compare French tétralogie.]
(Greek 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.
A group or series of three tragedies, exhibited together without a fourth piese, was called a trilogy.