I'd say ban the words 'sense of speed' ... people keep asking about this over and over again, and it drives me nuts.
I keep replying that the feeling of speed doesn't come from the graphics - compare that movie which puts GT4's RX8 on Tsukuba right next to a recording of a cockpit view of a real RX8 at Tsukuba. They look exactly the same. They have the same sense of speed. But nobody wants to add blur effects to the real life recording of a real car racing on the real track, yet people keep complaining about GT4.
The sense of speed doesn't come from what you see, but from what you feel. Do you feel the g-forces in your back, the struggle with the wheel to keep the car pointing the right way on the track when it over or understeers, the trembling of the car when the stiff suspension hits the kerbstones. A few months ago I was in a train doing over 300km/h and the only sense of speed you got was from looking through the side window and seeing what's really close to you whizz by. But if you're in a car, you focus on what's in front and barely notice what's on the side. However you do see more than on a normal monitor/tv or an in-car tv recording of a real car doing a lap on a real track. Therefore the best you can do to improve this 'sense of speed' (ugh) is to get a widescreen TV and a good Wheel with decent Force Feedback.
Combine that with a fast car on street tires and no TCS on in GT4 Prologue, and I say the sense of speed is pretty good. If that doesn't help, pick a track where the stuff on the side is real close to you like on Citta d'Aria, or also Grand Canyon with the people jumping away. It works for me. If it doesn't work for you, you're probably more the Burnout 3 type.