Positive:
+ Many cars
+ Many, tweaking settings very professionally represented as pressure levels of hardness, transmission and differential
+ Very highly detailed and extremely well-lit graphic
+ Many tuning parts whose performance increase is calculated for the car more realistic than in Forza 3
+ Engine sounds better overall than in Part 3
+ Online multiplayer available (big plus)
Negatives:
- Very unsettling sounds the collision of vehicles (sounds like as if you can come crashing a car with a tank)
- AI is very, very, very stupid: leaving almost never the racing line, driving an ever purely in the side. The braking point was set too early on the routes, so the AI only decelerates, can roll out and then only again in real braking point of the curve slows down again. That ride in the slipstream is really very difficult, because the AI too soon slows and you definitely reinfährt back. In career mode, you can not adjust the AI strength, so that one (R1) is challenged relentlessly until the last race series. The KI-starch seems to rise automatically with higher race series.
Summary: With the exception of the AI and the somewhat monotonous career, the game is really good and worth for each racing simulation Fan on the Xbox. In my opinion, the Xbox 360 racing simulation that approach is most likely to rFactor (for PC) when it comes to driving physics.