User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-9703010-20130914061755

'I have begun work on the fifth and final book in  The Heroes of Olympus. You will find out the title of book five when you reach the end of  The House of Hades (the same way I’ve revealed all the other titles in this series). The book will be released in the fall of 2014. Will that be the last story set in Percy Jackson’s world? It’s really too early to say, but I’ve got a lot of other irons in the fire.'

Hmph! He starts Book 5 and we poor beings are pushed down to staying with Three Books. UNFAIR!!!