GT4 is honestly the best racing game ever. Both Simulation and Arcade, as it includes both on the same DVD. If you've played GT3 and still have your game data saved on a memory card from it, then you can transfer up to $100,000 and license A and B over to GT4. So, starting the game with a "cheap used car" wouldn't apply to you. I started out with a Nissan Skyline GT-R R34, and basically dominated every race I was allowed to play in so far. Haha!
Not only can you tweak every aspect of your car, as someone has mentioned before, but there are so many ways to play; including playing on a LAN with others. (I belive up to 6 can play, haven't tested it yet, but I will soon) You can play A-spec or B-Spec, and Arcade, then you've got the "Las Vegas" Drag strip. (Which isn't the true 1/4 mile strip in Las Vegas, for those of you who don't know any better.)
The graphics are simply amazing to be a 4 year old console. Though, unless you've really played GT3 top to bottom, you probably wont notice much of a difference. The opening demo on GT3, in my opinion, is alot better than GT4. I think it had to do with the music though, because I'd leave my TV on with the demo playing 24/7...I just loved that song.
I can see "ricers" not liking the game though, as you actually have to know how to race a car to be any good at it, unlike games such as RidgeRacer, where you can make a turn at 200 MPH, or the fast and the furious type games, where you have "Stage 1" type upgrades.
Either way, GT4 seperates the men from the boys in true simulation racing; and they do it with class.