I've been mostly unemployed for the last couple months, so my only choice so far has been to rent GT4 instead of buy it. Unfortunately I had to wait a whole extra month to find it in at Blockbuster. But I think it was still well worth the wait.
First I tried the arcade mode, to get a quick feel. I tried the Ford Ka, Dodge Charger, Pontiac GTO (the new Holden Monaro-based one), and Lexus GS300 on Motorland, Autumn Ring, and El Capitan. One thing that caught me by surprise, when setting the options, was it didn't seem to want to let me set the accelerator and brake to the right analog stick like I'm used to from previous games. But once I got in the game I found it still works, so if anyone else still hasn't had a chance to play it yet, don't worry.
As for the cars themselves, the Ka is pretty slow as one would expect, and has a hard time getting above 5000 RPM, but still seems more fun to drive than a car of this class would have been in GT3 (or possibly even 2, due to the slower frame rate). The Charger is as fast and handles as "loosely" as one would expect for a car of that era. The new GTO feels about as fast but much more composed, as does the Lexus. I found that the so-called "GS300" has a turbo on it, which the real American version of the car doesn't, but I guess the Japanese version does. I don't think they should be giving these cars their American names unless the specs are based on the American versions... Just like in GT2 where the Nissan Silvias were renamed "240SX" even though they didn't have the 2.4 liter engine of the American version.
I noticed the physics puts more emphasis on front/rear weight transfer than it did in GT3, and contrary to reports I've previously heard, it is not impossible to make the car oversteer, even in the tiny front-wheel-drive Ford. (I wonder if some of them forgot to turn off TCS/ASM) I found it is certainly not impossible to drive the cars effectively with the Dual Shock 2 controller. It does take noticably more finesse than in GT3, 2, or probably even 1, but I still found it far more forgiving than PC-based simulations such as Grand Prix Legends, Racer, and Live For Speed. Which in my opinion it should be, on a console game designed for a wider audience who on average don't consider it as much of a "hobby" that they can justify the cost of constantly "investing" in bulky specialized hardware for (although it's good that the option is there for those of us with more time and money to spend).
Later I finally tried simulation mode, and finally decided on a white 1989 Mitsubishi Galant VR4 to start out with. I found the car handles nicely enough, but the stock gears are spaced too widely to keep the 2.0 liter turbo engine in its powerband. So my first upgrade will probably be the transmission. I played around on the Gymkhana course first, and was looking forward to seeing some cool footage (and taking cool photos) of me tearing around like crazy knocking over cones, only to find it won't let me watch or save a replay of the Gymkhana. I then tried the first Sunday Cup race, on Autumn Ring Mini, and placed third.
I'll try to post more details after I've had a chance to try it out more.