I really enjoy the Forza games, I've bought all of them since the original on the Xbox and I have over 100 hours and over 600 races in Forza 3.
I loaded up Forza 3 last night and played a few races. Even after one or two championships I was ready to take a break. I feel like I've already done everything I can, nothing is new or exciting anymore. From what I have seen Forza 4 is more of the same. There's some new features, like the Autovista which I don't have much interest in. Kinect support which I have no interest in. A newly structured career mode that from the looks of it just went from a calendar to a globe. They also say there's a new lighting engine, which looks like it just means they added a bunch of bloom.
I'm not bashing the game or anything, more Forza is good, but if I've lost most interest in Forza 3 is Forza 4 worth buying or should I pass?