What do you mean by more variety?
I heard the Skyline race car on the Nurb video, and it sounded fantastic. Though, some can't tell the difference, or said it still sounds fake, I heard a big difference and that makes me happy. The sounds of the vehicles in GT4 have improved to a more realistic level despite not sounding completely real.
The sounds in GT4 are not completely real, but the new addition, and the higher quality, of car noises made the game a lot more enjoyable to listen to. It sounds like PD has worked hard to make GT4 sound better, as well as look better.