When does the 4th Percy Jackson book come out?
or has it already?..
The fourth Percy Jackson book Battle of the Labyrinth was already released on May 6th, 2008. It is based on Greek mythology and the book is suitable for young adults. You can get it online at http://www.amazon.com/Battle-Labyrinth-Percy-Jackson-Olympians/dp/1423101499.
It has already come out. I think Rick Riordan has already started on a new series, which is part of the first one.
If you mean the original series Percy Jackson and the Olympians, then it already came out. But if you mean from the new series Heroes of Olympus, then it come out this fall. It's called "The House of Hades."