The Son of Neptune Audiobook

In The Lost Hero, three demigods named Jason, Piper, and Leo made their first visit to Camp Half-Blood, where they inherited a quest:

Seven half-bloods shall answer the call,
To storm or fire the world must fall.
An oath to keep with a final breath,
And foes bear arms to the Doors of Death.

Who are the other four mentioned in the prophesy? The answer may lie in another camp miles away, where a new camper has shown up and appears to be the son of Neptune, god of the sea.