regular sales, i think Forza 3 did worst than GT5 even in America.
in 10 weeks, Forza 3 sold 800K in America, in 10 weeks, GT5 sold 1.5m in AMerica.
Forza 3 got bundle so it sold a lots more after that.
as for Forza 4, nobody will know it till it come out. now it still don't have a release date yet. all i know is that there is not that much X360 gamers that like racing game that much. none of my friends on XBL play Forza 3. they either play a FPS game like CoD or Halo or Gears of War.
how dare you! refrain that statement at once. =D