What About Assists?
As you begin to develop your skills in Forza 4, you may want to take off some of the assists. This will allow you to have a more direct driving experience, and it has the potential to reduce your lap times. However, it's not wise to take off all the assists at once. So where should you start?
As I already mentioned, I would take the driving line and braking line away right off the bat, provided that you follow my advice and begin with the World Tour. As long as you learn with the slow cars, driving without the racing line should not be too difficult, and will help you build the skills required to play Forza 4 well.
Another assist that you may or may not want to adjust off the bat is the transmission. Automatic is the easiest mode, but also the slowest. Manual gives you far more control, and is preferable in all situations. You difficulty coming to grips with it will likely depend on what you drive in real life. If you've controlled gears before, it should not be difficult, but those who've always driven automatics will require a period of adjustment. I only recommend driving manual w/clutch if you have a high-end racing wheel with a stick manual transmission and racing pedals attached.
Once you start to feel in control of the vehicles you drive for the majority of the race, remove traction and stability control. These are different but related assists, and both help keep your car under control when things seem to be going wrong. However, they cut in early to achieve this, and so can rob you of some speed if you've become skilled enough to drive a car at its limit of traction without exceeding it.
Anti-lock brakes should be kept on until you're very comfortable with braking into corners. Without ABS, you can lock your brakes, which will increase your stopping distance and make your car harder to control. However, with very careful brake modulation you can brake a bit better without ABS, which will further increase lap times.
The last assist I recommend you remove is steering, which can be dialed over to "simulation." In this mode, you have no steering assists at all, which means your ever movement is communicated in full. I'm not sure if going to simulation steering actually has an impact on lap times, but I can say that it provides an even more realistic driving feel.