Because the trend is completely different at the start, it really isn't that logical to automatically assume the rest will behave like earlier versions did. Also there are some significant differences between San Andreas and GTA IV.

- GTA IV is the first GTA on the current generation consoles, making it more likely to be front loaded.

- San Andreas was launched at the end of October, so a completely different time of the year. I can see many people waiting to pick it up immediately because it could be used as a present during the holiday season.

- The userbase is a lot different. Many of the current owners will be avid gamers and more likely to pick up the game at launch

- Console prices are a lot higher at the moment than PS2 was during the launch of San Andreas, at least in the US (In Europe the Arcade 360 is a lot closer). Many people might have picked up a console to play the game at a price of under $ 200, but at the current prices the barrier to entry is higher. And by the time console prices come down there might be a new version of GTA out already.

It's possible that won't happen with the fourth GTA in 8 years, but it looks likely, to me.

 One could argue that it's not completely different. It's the same thing as it always was -- just even bigger, and even more popular. That's possible.

Realistically, though, games like this tend to get more fervent and fervent admirers, and stop growing over time. So that means larger launches, shorter legs. Historically, this is more likely, but definitely isn't a certainty.  

unlikely. Between the two the platforms it would have to have an outrageously high attach rate, and even in a couple years it would have to have a high attach rate, and still have enough legs to get there.

Plus SA was on PS2, Xbox, and PC, so it sold considerably more than 15 mil. I've heard more than 20 mil. So if in two years, the two platforms combined have an install base of 100 mil, it would still take a very high attach rate and incredible legs in order for it to reach SA levels. And in the new hype dominated HD market, there is a good chance the next big thing will cut the legs of GTAIV. After two or three more MEGA-blockbuster titles come out GTAIV will be old news.

I know SA was also on Xbox and PC, but I included only PS2 version cause it had already couple of times bigger userbase.

GTA games have always had enermous legs, and I don't think this is kinda game that gets outshined by any game, really. 

Btw. Where's every SSBB vs GTA IV topics? Do some people still think that SSBB will outsell both versions combined like few weeks back ago? ;)

@Bodhesatva: I didn't say the game wasn't similar, the differences I mention only focus on sales patterns and userbase.

I think it will pass III, but I don't think the userbase is large enough to top Vice City and San Andreas.

why not? it has a chance maybe small but a chance

I think it can outsell San Andrea's on the PS2. Not all the consoles combined though.

 No. As great as Vice City was with the 80's stuff, GTA IV demolishes anything Rockstar has done before by a long shot.