I would look on ebay.co.uk as there are many rs4 wheels in 18" but for looks i would recomend 18" OR 19" rs6 replicas they look so sweet on an S4.
also try
tyres
also try item number 7935027491 on ebay.co.uk for a special treat.
some pics of replicas on audis attached to help ya decide.
mark (a3 kid)