I doubt it. Coppola certainly could have gotten financial backing even in the late 1990s, and there's some evidence of at least the rudiments of a plot outline before Puzo's death. But Coppola has made it pretty clear he has no interest in making a fourth film, and frankly he's reaching the age where I doubt we will see much more output of any kind, but particularly the kind of work that is needed to construct as vast a production as a Godfather film.

If I had one wish, it would be to see a written treatment of the original Part 3 story; with the central plot being the final break between Tom and Michael. I think a fourth Godfather would just be universally depressing, more like a Goodfellas knock-off.