I don't own a hyped-up PC... err, XBOX, so I cannot say that I have legitimate experience playing even PGR1.
As much as I know, the "Project Gotham Racing" series is basically the old game series Rush. The Rush games (S.F., "Rush the Rock", Extreme Racing USA, and 2049), have been about unrealistic, usually dangerous racing where you can go off a bump at 150mph and (singing) "Fly like an eagle... into the future." Anyhow, PGR is mostly an arcade racer where you take exotic and not so exotic cars and trucks onto the streets of many famous cities the world over. Give or take the insanity of the Rush series, PGR is mostly like Rush.
But how do the two compare? PGR has a few cars GT4 won't have, including SUVs. PGR has some races in the wet, in which I am sure that you can configure a race to be wet. GT4 is a RACING game. It captures the pure thrill of GT road racing, sideways-style rally racing, and just the closest thing possible to doing that stuff in real life. GT been around longer than PGR. When a game has spawned a totally new perspective on racing games, other companies will try as hard as they can to emulate or better the best game. PGRs 1 and 2 gives you Need for Speed thrills without having to fend off cops or rush hour traffic.
So which is better? It depends on your tastes. If you want a pure simulation, GT4 is coming to video game stores near you later this year. If you basically want an NFS experience that's anything but, PGR is for you.
And again, I'm not trying to answer questions others perceive as dumb. I'm just expressing points to the best of my knowledge. If the insults keep up, this will be reminiscent of the "Grand Canyon View Debate" (if you are new to GTPlanet and don't konw what I'm talking about, keep looking on the GT4 Topics for the issue). Anyhow, points stated, nothing futher, I'm done.