any information about 4th Stage is completely rumor, and nothing more.

It would be nice to take a trip to Happogahara to battle the boys of Todoh Racing School. that'd be a great set of 13 episodes. Project D battling Todoh and the FF drivers of Happogahara. Sakai in his ITR trying and eventually succeeding in a trick to get Takumi to recklessly dive into a race against him.

Also you could learn about how Takumi does a lot better in battles now, than he does in solo runs. While Keisuke runs faster in the time attacks, but slower in an actual battle. All the while, Ryousuke is always Yoda-like and knows everything before anyone else does. I'd also love to see the race against the two dudes from tokyo in the S15. Mostly because it's an S15, though

I've yet to be able to read all the mangas, but I'm gonna pick up several and get the idea of what's going on from the pictures. heh.