What is the fifth series called in Meg Cabot books a mediator story?

There is no fifth series in Meg Cabot's Mediator books. The series only has four books:

1. The Mediator

2. The Boy Next Door

3. Shadowland

4. The Destiny of the Chosen

There are also a few spin-off novels that take place within the Mediator universe, but those are not considered part of the main series.