Instant Center. What are you thinking instant center is?
Instant center is the pivot point of the suspension lines when viewed from the side. Again, that depends on the geometry of your suspension. it depends on the angle of your bars/links in relation to each other. 3 link vs 4 link is going to be nearly the same instant center, assuming the lower links are parallel to the ground and the upper link(s) are set at the same angle.
This video explains it pretty well.
I think you've got a mistaken idea of instant center, your car will have 2, a front and a rear. its simply where the pivot points of the suspension meet.
Below are a few links to help you understand these terms a little bit.
On a leaf sprung car, instant center runs from the contact patch of the tire forward through the line from the front leaf spring mount. or, simply put, the Instant center is the the location of the front spring eye center...
Moving the leaf spring front mounting point down changes the instant center down and forward. The rear spring mounting point doesn't have much effect on the instant center. The length of the spring from the rear end to the front spring mounting point also affects the instant center, longer distance to the front eye from the rear end moves the instant center forward.
