Depends on when it is released. Grand Theft Auto V sold huge when the 360/PS3's install bases were at their maximum potential and just before the new generation. Grand Theft Auto IV was released only 2 1/2 years after 360 was released and 1 1/2 years after PS3's release. I believe if two Grand Theft Autos are released this generation (may not be likely), then the latter will sell more than the former. Whether or not they sell more than GTA V remains to be seen.