S04 has definitely gotten lost in the shuffle a bit with all the (deserved) hype surrounding the PSS. I can vouch for their excellent stability and steering response from our testing. The grip level is excellent. Not quite equal to the PSS, but better than most other tires in the category.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.