A tetralogy (from Greek τετρα- tetra-, "four" and -λογία -logia, "discourse") is a compound work that is made up of four distinct works. The name comes from the Attictheater, in which a tetralogy was a group of three tragedies followed by a satyr play, all by one author, to be played in one sitting at the Dionysia as part of a competition.
Antiphon of Rhamnus, an orator, taught his students with Tetralogies, each one consisting of four speeches: the prosecutor's opening speech, the first speech for the defence, the prosecutor's reply, and the defendant's conclusion. Three of Antiphon's tetralogies survive. In more recent times, Shakespeare wrote two tetralogies, the first consisting of the three Henry VI plays and Richard III, and the second consisting of Richard II, the two Henry IV plays, and Henry V. Richard Wagner's Der Ring des Nibelungen ("The Ring of the Nibelung" or "The Ring Cycle") is also referred to as a tetralogy.
As an alternative to "tetralogy", "quartet" is sometimes used, particularly for series of four books. The term "quadrilogy", basing the prefix on Latin prefix quadri- instead of the Greek prefix, and first recorded in 1865, has also been used for marketing series of movies, such as the Alien series.
"Markson’s writing was so informed by his reading—especially his late tetralogy (The Notecard Quartet, as those last four novels have been called) which is filled wholly with the cultural detritus he’d pick up from his voracious reading."