I was at the Playstation Experience, yesterday. (My impressions are coming shortly.)
If the demo I played was anything near what is going to be released then I am not surprised that it will be out so soon.
After 20 mins of play, I was convinced that the graphics engine was identical to the one used GT Concept. There may have been some improvements but I was not able to discern them. Also, with such a sort time with the game it was difficult to tell if the AI had been improved.
GT4 felt like GT2. The same as GT1 but with many more cars.
I hope I am wrong and that GT4 arrives with a stonking new graphics engine, but I will be surprised if it does.