Okay well I'm obviously I'm not Jack, but I don't think this is the END end of tftgs. Volume four is ending the arc of the whole pain god taking over the earth thing, not the end of all tftgs related stories! So don't worry... yet.
Jack must know Jeff Benzos because thats the only billionaire that has made medical tech for space travel.
Every good story has an end. Just like us - We have an end. It's called death.
But don't be sorry it's over, just... Look out for more books from Jack Townsend, or whatever pseudonym he tries to fool us with.