This game is just so incredibly hard to predict when it comes to sales. The previous three were monsters on the PS2 but a large percentage of sales came from people who spent less than $200 on their PS2. It's also one of the series to buck the trends and actually sell more copies in subsequent releases.
I think combined sales will easily reach 9 million lifetime... I don't know about 9 million by the end of October. We could easily see a scenario play out that's similar to what was seen on the PS2 - each successive release of GTA garners more sales as the userbase grows. Again, while that may make sense, this is the opposite of what most series experience, where the first iteration on a new console sells more copies than subsequent iterations for that generation.
My expectation for lifetime sales of GTA IV is closer to those of GTA III than GTA San Andreas.







