User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-86.28.255.73-20121016215252/@comment-5112532-20121017182137

I think that defies the whole point in the title of the 4th book - The House of Hades. Since it's on the mortal side of the doors, I don't think the rest of the 7 would be able to return to Camp Half-Blood, it would take too long to travel there and back and I doubt Hercules would let them exit or return to the Mediterrean Sea.

To be honest, I think that is all we can speculate! Assuming that it will just be Leo, Jason, Hazel, Piper, Frank, Nico and Coach Hedge battling monsters all the way to Epirius, Greece - sounds a bit boring to fill the whole 4th book!? :L