I think that if GT4 had online multiplayer, it still wouldn't touch PGR2. I am in the PS2 online beta trial, and all the games handle the online stuff differently. So with each game you get a completely different experience.
The thing I like with XBL is that it's pretty much the same for all the games, and for the most part that's good wholesome fun.
Saying all that, I am looking forward to playing GT4 with the FF wheel. I am really looking forward to taking the BMW M3 which I bought with my free 100,000 credits round a few tracks with it.
Also, have found a couple of BMW mods for F1C, so I'm hoping I'm going to be like a pig in the proverbial.