Try to learn to tame the car before adjusting your settings. Overall, you'll have better times and control if you do it this way. Settings CAN make up for where you lack (not a personal attack to you), but you're better off learning your car. In GT4 if you break traction and spin your tires, you'll be spinning them for awhile. Combine that with the high horsepower and you're asking for trouble.
Start easy. Learn how to half throttle. That way you'll have three positions instead of 2 (zero, half, full). If you're spinning tire, release the throttle a bit back to half. I imagine if you're having issues in a straight line, you'll be having issues in turns as well. When you hit the apex you can use half throttle until it straighten outs a bit more and then go for full.
Once you learn half throttle throw in a few more positions. 1/2 and 3/4 throttle. You'll begining to notice you can use the throttle sooner (but gradually) and get faster times and cornering speeds! Don't think this is just for analog users and DFP. I use the d-pad and X, it just takes a bit to teach your thumb to pressure buttons instead of mashing them. GL!
D-Pad Users Unite!