Random thoughts by Lisarenee - What can you call a series of books beyond a trilogy?

Have you ever wondered what to call books that go beyond 3 books, but not too much farther beyond? To me a series implies an almost never ending sequence of book. So I began to wonder are there terms that I could use to be more specific beyond the term trilogy? So I decided to search for a term which refers to a four part series. I thought if such a term existed it'd be quadrology, but when I typed it in to google I found the term quadrilogy. That lead me to allwords which states the following about quadrilogy. "Incorrectly formed from quadri-, by analogy with trilogy, and probably through ignorance of the word tetralogy." So I arrived at my answer, tetralogy. I was pleasantly surprised to find such an unused term. I also will be trying to slip this word into my daily conversations.  Here are some more terms you can use according to www.allwords.com: 

Related terms:
duology (2)
trilogy (3)
tetralogy (4) 
pentology (5)
hexology (6)
heptalogy (7)
octology (8)
nonology (9)
decology (10)

I must note that according to Merriam-Webster dictionary, however, pentalogy is only defined as a word referring to a medical condition with 5 symptoms. So who knows? Could I perhaps petition the above terms be added to the M-W dictionary?