It's not out for another five months, but it's just appeared on the various book sites. The fourth book in the Athenian Mysteries is The Marathon Conspiracy, written by some chap named Gary Corby, published by Soho Press (Random House).
I love the cover on this one. In fact, all of them are great. The covers are done by a very talented man named Stefano Vitale who lives in Venice. I the author am in Sydney, and the publisher's in New York. So an international effort all round.