My S4 came with a "chin" fairing on it, it's not the big belly pan that I've seen on some. It looks really good. I can post a picture if you'd like. As far as the fairing, I took mine off and it's not going back on...
anything that lives directly behind the front wheel is
probably more cosmetic. a little wind tunnel testing
could clear it up. the upper fairing / wind screen are
much more important for drag coefficient.
My lower cowl looks exactly like that above. Not sure about aerodynamics, but it looks cool -- gives the bike kind of a "complete" look. I bought it from a guy on this list for 100 bucks, a great deal.