Anyhow, GTA IV is a big game, its very popular based on name alone.
The only thing keeping it from selling another 5-10million copies is the price of the game itself, which is starting to loosin up a bit, since the console that has the game the xbox 360 is now 199.
I expect GTA IV sales will go up in the coming months, before holiday season ofcourse, and thanks to a cheaper 360, people will be getting either halo, GTA, or call of duty 4.