Forza 3 is already considered the better game when compared to GT5. If Forza 4 turns out to be an even better game....well PD are going to either have to step it up or just settle for becoming a bit player when they release GT6 ten years from now. By better game, I mean Forza 4 better have day/night cycles, rally racing, weather, AND keep every car a premium car. Also Forza 4 should get better balancing for its multiplayer. Right now, its too easy in Forza 3 to stack a fwd/AWD car with mods while being in the same class as an underpowered rwd car. Driving RWD cars is a valid life choice Turn 10, quit punishing us!
But above all, I seriously hope that whole premium car/standard car stuff becomes a side note when looking back at the GT series and will never be repeated again in any racing game series.