A reason why techno music is usually a good choice in racing games is because it kind of gets you into a game. Some say that some techno is generic because these "no-name" songs lack the energy and emotion of a heated race. The Wipeout series of games knows emotion and energy with its upbeat techno tracks. GT4 won't be entirely techno, but you can bet that the techno will exude emotion and get you in the racing mood.
Rock is a bit of a different story. You can imagine a classic rock tune as you rev up your vintage muscle car down the freeway. Rock music hasn't really been one of my favorite types of music, but rock transforms a heated race into a hardcore rock tune.
If GT4 has more jazzy music, then believe it or not, jazzy music will get you into a relaxed mood of driving. Of course, relaxed is NEVER the way to go, so it's usually used in the menu screens. I think more jazzy music will relax you before a race. Just the right melody will clear your mind of your worries prior to wanting to race a GT game.
This is a totally different approach to music, I know. But I kind of thought about the IMPACT of certain music, not so much the music itself. Like, listen to Ridge Racer V's techno tracks. They may be a bit cheesy, but the song from the Mad Capsule Markets called "Drftdvl" actually gets you into the mood of racing and racing fast.