te·tral·o·gy 
(tĕ-trălə-jē, -trŏl-)
(tĕ-trălə-jē, -trŏl-)n. pl. te·tral·o·gies
1. A series of four related dramatic, operatic, or literary works.
2. Medicine A complex of four symptoms.
[Greek tetralogiā : tetra-, tetra- + -logos, word, saying; see -LOGY.]