tetralogy (n.)youdaoicibaDictYouDict[tetralogy 词源字典]
1650s, from Greek tetralogia, from tetra- (see tetra-) + -logia (see -logy). A group of four dramatic compositions, originally three tragedies (the trilogia) and a Satyric play.[tetralogy etymology, tetralogy origin, 英语词源]