There was over 1 Million copies of Forza 3 bundles in the Americas during the 2010 Holiday season alone. It was also bundled through '09 Holidays and throughout mid 2010.
I would say a fair guess at bundled sales of the game are over 1.5 Million units, in Americas.
If Forza 4 recieves the same treatment (Hint, it will. Forza 2 AND 3 have both had it), it will easily outsell GT5 there.