S4 put the transmission into a more sporting mode. It is supposed to upshift/downshift a bit quicker and more responsively (seemed to work too). S3 does the same thing, but locks out the overdrive so the car only runs in the first three gears. Very useful for keeping the engine in its powerband, such as when driving hard through corners on a twisty roadOne of the best cars I've ever owned was my 89 Legend. Too bad I wrecked it.