No, there is no 'Godfather IV' movie being planned. The Leo DiCaprio dealy was just online speculation about who could play a young Sonny.
What is true is Random House and the Mario Puzo estate have given the greenlight for author Mark Winegardner to pen a fourth 'Godfather' novel. Paramount Pictures is just waiting on the finished product. The working title is 'The Godfather Returns'. Personally, I think it should be 'The Godfather - The Continuing Story'.