Well the fact that it's about umpteen times more detail. I'm not saying the Gt4 one looks bad, but in that shot, forza does look better tbh, I'm not saying it does normally, because it doesn't, but that's a VERY flattering shot.
Cobra you simply don't get it do you. GT4 looks like it does because of ART DIRECTION. Technically it's pretty damn good too, smoother graphics, fast frame rate, but the textures are pretty lousy, the audio is weak, hell it can only have ONE of some cars on track at once. PD are masters of getting great looking graphics out of weak systems. Forza looks like it does because A. the devs aint as good as PD in the art direction, 2. It has to do stuff like damage, decent audio, more complex physics that take in to account stuff like tyre heat etc, then theres the stuff that breaks off the cars, more cars on tracks blah blah blah, all adds a performance hit. The xbox is hardly a super a computer.
GT4 is relativly plain in some ways, cars have no damage, no skid marks, low quality textures, low quality sound and only 6 cars on track at once (1 car in some cases), which means they can run it at a silky 60 fps, and it looks freaking great.