Unfortunately, polygon figures are something I've had trouble finding for Forza Motorsport 3, and after all this time they're still elusive, so I don't expect to see them for FM4, either. The most I ever really saw for FM3 was that the cars were modeled with ten times the polygons used in FM2, which indicates a huge improvement from FM2 to FM3, but to what actual figures?
However, I did just now run into an article saying that FM4's cars are rendered with over a million polygons, and that this is up from 400,000 in FM3. The same article says GT5's cars were modeled with 500,000, so if these numbers are accurate then it means FM3's cars were almost equal in detail to GT5's cars in a game that came out a year later, and FM4's cars are built with double those of GT5's "Premium" cars.
http://drivinggamespro.com/featured/forza-motorsport-4-cars/
However, upon further reading of that article, they seem to be confused about the detail of FM4's cars because they believed "The high poly count makes sense given that you can open every body panel via Kinect", which is obviously in reference to Autovista, and the Autovista models aren't the same that we race and most of the cars don't have an Autovista model.
