The term octology was introduced 8 March 2010 by Dr. Andrej Poleev in his book of the same title to denominate a new scientific discipline, which should unify morphogenetic linguistics and neurobiology to investigate the development of the words, cognition and behavior.
Reference: A. Poleev. Octology, 2010, ISBN 9781007171184. URL: see related links.
Although the term is not in common usage, the appropriate term is "tetralogy," which has the same derivation as "trilogy," for 3 publications. For what it's worth, "Tetra" means 4, and the suffix "logy" refers to a publication.
This is an excellent question. The terms are as follows: Set of 2 - Duology, 3 - Trilogy, 4 - Tetralogy, 5 - Pentalogy, 6 - Hexology, 7 - Heptalogy, 8 - Octology, 9 - Ennealogy, 10 - Decology. After the 10 mark there is a slight change... 11 - Undecology, 12 - Dodecology... from 13 to 18 the same greek prefixes are used before the word part "decology" ~ tri,tetra,penta,hexa,hepta and octodecology... on to 20 it is Nonodecology and Icosology. Hope this helps and answers your question.