Why is The Twilight Saga called a saga if its only made of four books?

The term "saga" is the term for the old Icelandic stories about ancient Scandinavian and Germanic history, about the early voyages of the Vikings, about immigration to Iceland and of feuds between different Icelandic families. How many volumes it has, has nothing to do with it.

Stephenie Meyer is incompetent enough to dub her series a "saga", not knowing what the term "saga" means. Twilight is not a saga, and never will be. It's a series.

Hope that helps! :)