As the author mentions that he's partly basing the book on Puzo's unproduced screenplay for GFIV, there's some small hints here, in a FFC interview from 2007:

MTV: "Godfather IV" was talked about, it seems, at one time. You and Mario Puzo — is this true? — went to Paramount and said, "We're interested, we'll do it," and they said, "We're not interested."

Coppola: I never thought making a second "Godfather" made sense to me. ... I thought the end of the first "Godfather" film was the end of it. Michael has become what he's become. He's paid a terrible price for it, and that's the point. The last shot of closing his wife out was the end. So when they wanted a second "Godfather," it was just to make money. ... It was the beginning of this franchise mentality, so I resisted it. ... But I was working on an original idea of ... telling a story of a father and a son at the same age ... two stories paralleling. They prevailed on me so much that I said, "Well, I'll do it, but I'll have total control, and I'll make it be this story and work it into 'The Godfather.' " When that was done — miracle of miracles that it was well-received; like anything, it could have gone as bad as gone right — then I was done with it.

Many years later, after "One From the Heart," after [accumulating] huge debt, unbelievable debt for a young guy, the chance to do "Godfather III" was a chance for me to get out of my problems, and I did it as best I knew how. ... And then there was talk of a fourth "Godfather." And I had an idea of how you could do it, oddly enough, again paralleling two stories because it was a big part of the book that had never been made — it was the period sort of between the old period in "Godfather II" and when you see Marlon Brando in "The Godfather." Mario called it the "happy years" — when we killed them and they didn't kill us. [He laughs.]

And Mario was very concerned to make money because he was getting older and he really wanted to leave his kids well-fixed, and I said to Paramount, "Look it, we have an idea of a structure of this thing. Pay Mario Puzo a million dollars to do this first draft, and I'll help him and work with him. You don't have to pay me anything. But he's getting old, and he's not entirely well." And they basically didn't do it. And then he died.

http://www.mtv.com/news/articles/1576410/francis-ford-coppola-on-godfather-iv.jhtml

The Family Corleone, aka 'The Godfather Part IV: The Happy Years'?

Amazon listing:



Publication Date: May 8, 2012

New York, 1933. The city and the nation are in the depths of the Great Depression. The crime families of New York have prospered in this time, but with the coming end of Prohibition, a battle is looming that will determine which organizations will rise and which will face a violent end.
For Vito Corleone, nothing is more important that his family's future. While his youngest children, Michael, Fredo, and Connie, are in school, unaware of their father's true occupation, and his adopted son Tom Hagen is a college student, he worries most about Sonny, his eldest child. Vito pushes Sonny to be a businessman, but Sonny-17 years-old, impatient and reckless-wants something else: To follow in his father's footsteps and become a part of the real family business.
An exhilarating and profound novel of tradition and violence, of loyalty and betrayal, The Family Corleone will appeal to the legions of fans who can never get enough of The Godfather, as well as introduce it to a whole new generation.