First of all, don't associate rap music with the tuner car crowd. It's popular, but doesn't denote "rice." Besides, what rap star probably wouldn't want a 1963 Chevy Impala? So, go easy on that.
The big factor for all of this is, how can you appeal to as many tastes of music for GT gamers as possible? I think the GT series has sort of evolved into a way that almost anyone can find anything they like. The hip-hop selection wasn't as diverse in GT3, but I'll take Snoop Dogg and from the Wu-Tang Clan, Raekwon. The big battle in terms of what America would WANT to have in GT4 is a hip-hop vs. electronica vs. rock Triple Threat. The best course of action would be PD coming up with about as much music as possible for the three popular genres of music. Because let's face it. In the game, I don't think an average person would want to listen to Toby Keith while racing. Average person might not want to hear people like Vanilla Ice, Keiko Matsui (a Japanese jazz pianist with inspirational and beautiful music), Sarah Brightman, Charlotte Church, the Trans-Siberian Orchestra, or anything like that. I say "account for everyone," and as long as a big majority of people dig a certain type of music, game teams will focus in on that.
So here's my question. GT4 will likely feature rock, hip-hop, and electronica (trance, techno, industrial, etc.). If GT4 included one other genre of music, what would you want it to be and who in that genre would you recommend?