One thing to keep in mind is that the Gran Turismo kind of deals with a broad spectrum of automobiles while games like the Rallisport Challenge series zero in on one specific form of motorsport. Gran Turismo isn't so much of a do-it-all racing game, but it's surely a new challenge since GT2. I think the Cultus Pikes Peak (who remembers this car when the Escudo completely overshadows this one?) was more livable with 700 hp, working lights, and didn't really seem all power, no handling.
As far as Gran Turismo 4 goes, PD wanted to start anew. This deal here is pretty tough. Roadways are narrow and require more concentration. Not that I've done any, but I think this is more like Autocross racing. Usually in Autocross racing I've seen on TV, you have a one-lane road, and your job is to navigate the course effectively without mowing down the cones. Most of the older GT rally tracks are still in the game (without Pikes Peak, I know). I seen Swiss Alps in the Arcade Mode track listing, and you can still have fun with that. But for sure, if you're not ready for rally racing, don't go with Grand Canyon. I guess to prepare for Grand Canyon, try Tahiti Maze and master it.
In closing, I'm going to be honest. One of my biggest weaknesses in rally racing is attacking apexes. Sometimes, I drift out and don't usually attack the deepest part of the apex without hitting the wall. As a result, my lap times suffer from not cutting into the deepest part of each apex of each turn in rally. That's my biggest weakness. But definitely train up for rally racing in GT4.