All I know is that the demo of Forza that is out know is uncompelling, but given that it has a few months to cook, maybe it will turn out differently, although I doubt it.
Basically, GT4 seems to be GT3 + more tracks + more cars. Pretty good in my book as the gameplay and controls are spot on. No online sucks the big one and the ability to bounce off of other cars and walls is still there.
I'd expect GT4 to get mid 8's in most reviews as it hasn't really changed the formula from 2001, which was preonline console gaming and that just doesn't meet the minimum requirements any more. I know that Kaz won't do it unless it's superior quality, but it's still missing.