How about the front end? Suspended? If not, you certainly don't want to go to a 4link. Consider the benefits of an ARB when added to a fully suspended car. The ARB acts to increase the rear roll stiffness relative to the front roll stiffness. This results in more of the reaction to the driveshaft torque being fed back to help equalize the rear tire loading.
So, instead of a 4link at the rear, you might be far better off to go to a softly sprung front end.
http://www.racetec.cc/shope