I'll put here a small suspension tuning guide that can be applied on GT4. there's more than this in it, but those things can't be adjusted in GT4. They can be adjusted in games like LFS, GPL and GTL, etc..
SPRINGS:
Stiffening front end =
Understeers in corners.
More grip, especially when exiting the corner.
Less traction on uneven surfaces.
Front tyres wear out faster
The car is easier to control
Front dips less during deceleration
Stiffening rear end=
Oversteers in corners.
Less grip, especially when exiting the corner.
Less grip on uneven surfaces.
Rear tyre wear increases.
Easier to control
Stiffening both =
Ride height can be dropped so the car gets underbody wacuum that glues it to the surface of the track.
Less grip on uneven surfaces.
Rear tyres wear more.
Easy to control.
Softening the front =
Oversteers in corners.
Less grip, especially when exiting the corner.
More grip on uneven surfaces.
Front tyre wear decreases
Controlling the car is harder
Could lead to rising the ride height at front.
Front dips more during deceleration
Softening the rear =
Understeers in corners.
More grip when exiting the corner.
More grip on uneven surface.
Less tyre wear
Controlling the car is harder
Could lead to rising ride height at rear
Softening both =
More grip on uneven surfaces.
Less tyre wear.
Car is harder to control.
Could lead to rising the ride height.
Ride height
Lowering the front - oversteers in corners.
Lowering the rear - understeers in corners.
Raising the front - understeers in corners.
Raising the rear - oversteers in corners.
Anti-Rollbar/Stabilizer
Stiffer front stabilizer =
Understeers in corners.
Less grip on uneven surfaces.
Easier to control the car.
Stiffer rear stabilizer =
Oversteers in corners.
Less grip especially when exiting the corner.
Less grip on uneven surfaces.
Rear tyre wear increases.
Easier to control the car.
Stiffening both stabilizers =
Less grip, especially when exciting the corner.
Increases tyre wear.
Easier to control the car.
Softer front stabilizer =
Oversteers in corners.
More grip when exiting the corner.
Less front tyre wear.
Harder to control the car.
Softer rear stabilizer =
Understeers in corners.
More grip on uneven surfaces.
More grip, especially when exiting the corner.
Less rear tyre wear.
Harder to control the car.
Softening both =
Could lead to raising the ride height.
More grip on uneven surfaces.
Less tyre wear.
Harder to control the car.
Bump dampers
prevents the wheel from moving upwards during bump.
Stiffening the front =
Understeers at uneven corners.
Increases front tyre wear.
Stiffening the rear =
Oversteers at uneven corners.
Less grip on uneven surfaces.
Increases rear tyre wear.
Stiffening both =
Less grip on uneven surfaces.
Tyre wear increases.
Harder to control the car on uneven surfaces.
Softening the front =
Oversteers on uneven corners.
Increases tyre wear at front.
Softening the rear =
Understeers on uneven corners.
More grip on uneven surfaces.
Rear tyre wear decreases.
Softening both =
More grip on uneven surfaces.
Less tyre wear.
Rebound dampers
prevents the wheel from moving downwards during bump.
Stiffening the front =
Understeers at the entrance and exit of the corner.
Increases front tyre wear.
Stiffening the rear =
Oversteers at the entrance and exit of the corner.
Less grip at the entrance and exit of the corner.
Stiffening both =
Easier to control the car.
Chassis is more stabile.
Difficult to find correct suspension balance.
Softening the front =
Oversteers at the entrance and exit of the corner.
Less tyre wear at front.
Softening the rear =
Understeers at the entrance and exit of the corner.
More grip at the entrance and exit of the corner.
Less rear tyre wear.
Softening both =
Car is harder to control.
Chassis rolls.
Springs have greater meaning in corners.
Suspension balance
Front-biased =
Understeers while braking, on the entrance of the corner.
Front wheels could lock up.
Longer braking distance.
Rear-biased =
Oversteers while braking, on the entrance of the corner.
Rear wheels could lock up.
Longer braking distance.