hi i'm just making a comparison between gran turismo 5 and forza 4 but don't blame me all are just my opinions.
in gameplay part especially single player and story(main game is also good) forza tried to use your develops and experiments on playing eventhough the car is important but GT used as your car and knowledge of cheating and subways (like moving to the forward place from grasses) although in special events forza has completely imitated from GT as top gear classes and Jeremy's talking at the first of the game but tutorials and practices are much more better.
in graphic part GT made it really good but in order to make the game faster and release it for fans they made out some laps and unacceptable mistakes like as standard cars and shadows but forza made it better, a lot better, but forza just copy the graphic from GT and ordered the pixels and particions so it was better.
voices and sound were a lot better classic musics in GT are awesome but they don't have any interests in races as forza made it fixed.
by the way I thinks it's enough all I was trying to say was that GT 5 was really great but some unforgivable parts as forza was much more better but likewise had some bad mistakes as it imitated GT 5 in some ways but in better forms.