GTA4 is the game I think most people are seriously over-estimating. The last few parts did very well because they were games everyone could play on the console everyone had. I've typed a long explanation for this a million times, which I have no intention of repeating here, but basically most people have trouble playing a game where you shoot aliens, lead armies etc, but almost everyone can relate to a regular guy on the street, getting caught in a life of crime. I don't think the hordes of people who bought the really succesful ones on the PS2 generally have HD consoles, and GTA4 will sell 5 million copies during its lifetime at best. That's still very well, but definitely not the best selling game of the year. Of course I never liked the GTA games, so I don't exactly understand the appeal in them. We'll see if I was right, or very wrong soon enough.
Halo 3 is likely already past 7 million copies sold now (nevermind all the sales for the next 2-3 years for the game) and you think GTA4, a sequel to games that sold over double what each Halo games sold, is going to sell less than half of Halo 3 in it's lifetime? That it's going to sell just a few hundred thousand copies more than Grand Theft Auto: Liberty City Stories sold on the PSP, a system known for having weak sales compared to the consoles?