What is the name of the fifth Heroes of Olympus book?

The name of book number 4 in the Heroes of Olympus series is called: The Demigod

Diaries.

No you fool. It isn't the fourth book in the series. It's just a extra like the Demigod Files or Percy Jackson and the Olympians: The Ultimate Guide. Besides, The Demigod Diaries comes out before the third book: The Mark of Athena. The last guy who tried to answer this doesn't have any clue what he's talking about. According to my sources, the fourth book might be called The Dawn of Gaea. I'm not completely sure though.