Matt Farah said(in his podcast) that he couldn't tell the difference between PSS and PS4-S's. It's probably a very slight improvement over PSS's. I would buy the tires you can get the best deal on. Just make sure the build date is not older than 6 or so months. Tires are really only good for five years from the date they are built.
http://shoutengine.com/TheSmokingTir...n-racers-32319