Well, book 4 is done. At least the rough is. I'm now on book 5, which is a historical piece on Earthfleet. It tells where the fleet came from and how they got started.
I'm writing book after book in this series back to back for the purpose of getting all the ideas down before returning to do edit work so that I have as much of the master story done as possible when it's time to go back, clean up, and then publish book 3.