Stray conversation has a way of sparking curiosity. When a passing comment asking what a series of four books was called, I had to wonder--just what is it called? Since a trilogy is three, would a quadrilogy be four? And, guess what? It is--in non-standard usage. Goggle is full of references to the Alien quadrilogy. The real word is tetralogy. Look it up! So what about five or six or seven. . . Here's a handy list for you. All the words can be found on the net in dictionaries like Merriam-Webster, Wiktionary, or allwords.com.
tetralogy-a series of four connected works
pentalogy-a series of five related works of art (hey, books are art!)
hexalogy-Anyone seen the Star Wars hexalogy?
heptalogy-a series of seven creative works connected by a common storyline--Harry Potter for instance
octology-a collection of eight books as one single unit
nonology-a trilogy of trilogies
Now, I know most of us would like to create at least one novel, but as my witty husband quipped, "The word for five is millionaire!
Happy Writing--ZMS =^..^=