This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A series of four related dramatic, operatic, or literary works.

n. Medicine A complex of four symptoms.

n. A set of four works of art that are connected, and that can be seen either as a single work or as four individual works. They are commonly found in literature, film, or video games.

n. A combination of four symptoms.

n. 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.

n. A group of four dramatic compositions, three tragic and one satyric, which were exhibited in connection on the Athenian stage for the prize at the festivals of Bacchus.

n. a series of four related works (plays or operas or novels)

"I can imagine someone saying that the New Sun tetralogy is not to his taste, or that some events in the books were unclear at first reading, but you have cut yourself from some of the finest writing in the English language, certainly the finest in science fiction, if you dismiss this wonder as crap."

series    serial   

ennealogy    decalogy    octalogy    hexalogy    pentalogy   

quadrilogy   

bullfiddle    narwhale    holo-room    diatom    libretti