Theres no point in talking about Forza 4 in this thread. Its quite likely we won't see another one for this generation. My guess is Turn 10 are doing Project Gotham to coincide with Xbox Natal and then after that Forza 4 will come after a 3-4 year development window on the Xbox next sometime like 2012/2013 and be a launch/2nd year title 2012 or 2013.
Sorry for the offtopic but it doesn't make sense to even consider a Forza 4 for this generation, or even that it will be compared to GT5 and not say GT6.