I hope you dont mind me pm'ing you.
I've recently been having some leaking from my windscreen washer bottle and have searched and posted on the forum and got some very helpful replies. I have just done a search on google and it has brought up a an Audisport post from you with a similar problem but you haven't mentioned if you sorted the problem out and I just wondered what the final outcome was. If you have the time and can remember what the fault was, how you repaired it etc I would be very grateful.
Could be a common fault, I have an S4 too. Great cars. I think I am hoping it is the rubber seal between the top and bottom tanks as this looks like the easiest fix.