Forza 3: 2,725,971
GT5 was doing better than GT4 in America... so maybe can sell near 3 million... Forza 4 will sell better than Forza 3... so near 3 million too.
I guess Forza 4 will sell better if it had Kinnect support... over 3 million.
As far as I know it will have Kinnect support also it will be heavily bundled. Maybe not as much as Forza 2 but more then Forza 3.