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.
After a heptalogy, which consists of a series of seven works or literary pieces, the next logical sequence would be an octalogy. An octalogy is a series of eight works or literary pieces. This term follows the pattern of numerical prefixes, with "octa-" meaning eight. Each work in an octalogy would continue the storyline or theme established in the previous heptalogy.