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 Attic theater, 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.
[1] In the early modern period of literature, Shakespeare drafted a pair of tetralogies, the first consisting of the three Henry VI plays and Richard III, and the second, what we now call a prequel because it is set earlier, consisting of Richard II, the two Henry IV plays, and Henry V.[6] As an alternative to "tetralogy", "quartet" is sometimes used, particularly for series of four books.
The term "quadrilogy", using the Latin prefix quadri- instead of the Greek, and first recorded in 1865,[7] has also been used for marketing the Alien movies.