Difference Between S4 & A4

Hello to everyone, I am new to the Audi forums.

I just sold my car and am looking into getting an Audi, preferrably the S4. I'll be looking at a 2001 S4 this weekend. I was wondering what the difference between an S4 and an A4 was visually. I know the engines are different and I've heard they share the same chassis. But if I were to go with aero styling (body kit, headlights, etc) would the parts be the same between the two? Because all I see are parts for the A4. Thanks in advance!