That depends mostly on what aspect of the game you enjoyed from GT, GT2, or GT3.
There's more tracks and more racing. There are over 700 cars, and many more new brands and classic cars. Visually, it's better than the rest, but to me, as long as I don't get sick from playing a game, the graphics do not matter to me (I grew up with an Atari 2600).
I've played them all, and GT4 is probably my 2nd favorite, after GT2. The biggest problem for me is the AI (computer cars), which is not much better than it was in GT3, although they make smarter pit decisions compared to GT3. There's also some funny restrictions on certain cars that is annoying, and a lot of fluff added to the game that doesn't affect the basic game-play (Photo Mode, B-spec, for example).
If you're contemplating a purchase, and you've enjoyed the other three games, than GT4 will not disappoint you.