100% agree with JRL, you should replace in pairs. I don't know how someone can claim the 4c system compensates for different shock heights and response rates between one good new and one used. Nothing in the way I've seen it's designed would make me think that was possible. It's a simple semi-active suspension system which only adjust dampening rates of the strut based on state information like ride height, accelerometer, steering angle, braking force etc. I don't think it has a feedback to match rates between an old worn strut and a new one.