When thinking about music, I think techno is pretty cool for music. You can have cool techno like in the WipeOut series, or you can for the kinds of techno Namco has in its games. Namco's techno has a lot of drum beats. Listen to Ridge Racer V or Tekken Tag Tournament. Now, RRV has some real artists. Even though some think RRV has the second best music (RRT4, whose music is mostly jazzy) in the Ridge Racer series, I think something rhythmetic would work perfectly.
Also when thinking about music, think about who would come in an do exclusive songs for GT4. For example, Snoop Dogg came up his "Dogg's Turismo 3." When I heard that song, I thought it was pretty original, not some cheesy, nonsense-making generic music. And who knows? Maybe there will be music for many kinds of people. "Across the Nation" from the Union Underground is the theme song for RAW, but KIND OF sounds cool for GT4. Japanese jazz pianist Keiko Matsui has some music in case you want to take a nice, quiet drive. And who knows? Charlotte Church may sing her heart out in case you have Italian muscle, and you're zooming at 175+ mph.
So, anything can happen. We just have to wait and see. Speaking of which, maybe Japanese pop stars will finally get their spotlight. There are some Japanese pop artists whose music would be cool to listen to, even if you cannot translate (like me). Examples would include:
Every Little Thing - Kimochi
Ayumi Hamasaki - Kanariya (Power Mix)
Aiko - Anata to Akushu
Exile - O'ver
m-Flo - Come again
I'll reply later with more on Japanese music. stand by...