If a GF IV movie was based in substantial part on the original novel (the 'Happy Years' 1927-1941), and based almost entirely on characters already introduced in fully-accepted canon works (the three films and Puzo's original novel), I would consider it canon.

If FFC produced or directed a new film based on the Winegardner novels I would consider it 'semi-canon,' but I cannot believe he would do so, since the main point of the Winegardner novels is to marginalize (or write out) the characters created by FFC and Puzo in favor of the Nick Geraci Comedy Show.

I would be astonished, if FFC elected to be involved in any new GF production, he would not order a new script based entirely on the characters originated by himself and Puzo.

If Winegardner can sell a Nick Geraci movie, more power to him. But it has little or nothing to do with "The Godfather."