Glisse

I understand, but 225/45 R18 or 245/40 R18 (sidewalls are the same height within a couple of millimetres) seems as good as you can get on the S4. Using the (higher) factory recommended winter pressures will help to avoid wheel or sidewall damage, as well as keeping the tread pattern open. In Europe, the primary reason for choosing the 225/45 R18 on 7.5J x 18 wheels would be to enable fitting of snow chains. The 20mm difference in tyre width is going to be secondary to the differences between types of tyre. But kudos to you for giving this plenty of thought - the quattro system can only provide traction up to the level of the tyre itself, and no more than that.