What is it called when a series has 4 books?

There's no specific term for a series with 4 books. You could simply refer to it as a four-book series or a quadrilogy.

While "quadrilogy" is technically correct, it's not commonly used.

Other options, depending on the context, could include:

* Tetralogy: This term is used for a series of four works, particularly in music or literature.

* Series of four: This is a more straightforward and common way to describe it.

Ultimately, the best way to describe a four-book series depends on the specific context and your preference.