RajDatta

Bruce, for the price for S4 calipers, you can upgrade to big reds so there is no value there. Its one thing if you are getting M030 spindles with it, but otherwise you will need adapters to make it fit.
I have used porterfields in the past with S4's but with my current setup, I went back to stock. I had porterfields with slotted rotors and that car stopped on a dime.
Ideally, stay stock unless you are either doing high performance or either you cannot stand dust from stock pads.