It is typically called a cycle.
A series consisting of 4 books is typically referred to as a "tetralogy."
The series of books is called the Chronicles of Narnia.
There are 4 books in the series.
A series of 10 books is commonly referred to as a decalogy.
13 books are in the series
a series of books about a fantasy land called Narnia. I have a copy of one of the series
Decalogy
A series.
A series of the same books on the same theme by the same author is called a book series or a book franchise.
4 lemonade war lemon book
A quartet.
Trilogy
There are four books in Stephanie Meyers Twilight Saga.#1-Twilight#2-New Moon#3-Eclipse#4-Breaking DawnNote: There is a book series called a Saga it will always contain four books.