Some cool new features, but not a huge improvement over GT3.
The are a lot more cars in this one than the past games, there's something like 700 cars. Pretty much every important car company is in here except for Ferrari and Lamborghini, which sucks, but there is still a great selection of cars. Even the Ford Model T is in the game.
Most of the made up tracks from the previous games are in here like Trial Mountain and the High Speed Ring. There are some other great race tracks like Laguna Seca, Nurburgring, and Circuit d'Sarthe. The rally race tracks are awesome, especially the Grand Canyon where the spectators are actually out on the track taking pictures of your car and then they run back as you get closer to them.
The simulation mode is pretty much the same, you start off with enough credits to buy a used car. After winning races you will win prize cars and earn credits to buy upgrades and better cars. There are many different types of races like FF races, 4WD races, tuner car races, and other car-specific races. Events are organized into beginner, intermediate, pro (etc.) levels. You will need to acquire licenses for some higher ranked events. There are also new driving missions which are challenges like pass all cars within 1 lap, use slipstreaming to overtake your opponents, or perform a pass on a difficult corner, etc.
In the arcade mode you can create races by choosing a car, track, and race settings. There is also a 2 player splitscreen mode.
Overall, Gran Turismo 4 is a great driving sim. I recommend it if you love cars. The only bad thing about this game is the A.I., they just drive a set course and they slam into you if you get in there way and they don't try to drive around you if you block them. It makes the game boring sometimes, but atleast they fixed this problem in GT5 Prologue, so GT5 will have better A.I.