There is an interesting article on the net (www.jgeoff.com/godfather)on the topic of Godfather IV.

Here's an extract from the article which I basicly chose at random. It was taken from some Hollywood magazine article which was written on 21st June 1999.But of course Mario Puzo died on the 2nd of July which basicly brought about the end of any plans there might have been to make Godfather 4 :

"Discussions are under way between Mario Puzo and Francis Ford Coppola to bring the fourth installment of "The Godfather" saga to the screen with actors Andy Garcia and Leonardo DiCaprio already on board, several sources have confirmed. Although the story line is not known, sources say that the structure is in the vein of "Godfather II" in that it will incorporate flashbacks and tell two different stories set in the past and present, and that Garcia and DiCaprio would never appear in the same scene. DiCaprio has suggested that he might play the young Sonny Corleone (portrayed originally by James Caan), which would then make him Garcia's father, but that is only a possibility. Puzo and Coppola will decide the ultimate story line. While a deal for a Puzo script is currently being hammered out, Paramount toppers and the two actors are said to be crossing their fingers in hopes that all deals will be consummated. Coppola and the studio have danced around a fourth installment for the past five years. It is only recently, however, that headway has been made in bringing the project to life. Garcia, a friend of both Coppola and DiCaprio, was said to be instrumental in bringing the components of the project together.

Coppola, who spoke to The Hollywood Reporter on Friday, was cautious about the fourth installment but acknowledged that actors' reps have been in talks with Paramount about "The Godfather, Part IV." Puzo is also actively developing the story. Although several sources confirmed Coppola's involvement, the filmmaker remained coy. On Friday, Coppola said that his attention currently is on another project. "