The above information is incorrect.
GT4 makes use of a dual-layer DVD b/c the game itself is over 9 GB. Please do not say that the game is 3.5GB or any other size unless you are absolutely sure of your information, just don't want more rumors/misinformation to be floating around. I know that people will want proof of GT4 really being as large as 9GB, so I have provided some in the following paragraphs. The following excerpt is from an IGN article from Nov. 9, the day Gran Turismo went Gold.
"
At the party, Polyphony president Kazunori Yamauchi gave out final stats for the game, including some staggering numbers. Some of these were announced before, but some of them weren't, so listen up! First, GT4 really is huge. The game makes use of a dual layered DVD, totaling, Yamauchi revealed, over nine gigabytes of data."
Here's the link to the actual article as well.
IGN Article
As the article clearly states, Kazunori Yamauchi himself said that the game totals over 9 GB of data. If he was speaking about the capacity of the disc, he would have said so, but he didn't. I would greatly appreciate it if everyone would cease to post false information while claiming it to be fact. Please, please make this forum less cluttered with misinformation by only posting what you know to be true in cases such as this. Thanks.
*EDIT: Gabkicks is right, there is no need for name calling. Took off my grumpy pants and made it a nicer, more civil post.