I ventured back to Forza 3 a few days ago just to get my fix of Porsche's! There were a few things that immediately struck me that I hadn't really noticed while playing through the game in the past.
First, the physics... FM4 is a vast improvement there. It's really noticeable when you're flying around a tight corner in FM3, and you exit the corner at a higher speed than you should. The car doesn't (naturally) push back and throw you off your line (the glue on the tires analogy).
Second, the lighting... Now that I've played FM4, the lighting in FM3 seems really dark. I had noticed that problem during my first play through of the game, but not to this extent.
Still, being able to drive the Porsche's is reason enough for me to pop in the FM3 disc from time to time. Even with these noticeable differences it's still a great game!