Quattro tyre requirements

[Phil3103]

All

In the process of buying an S5. The one I want is perfect other than it's got two new front tyres (no probs there) but the rear ones haven't been replaced and only have 4mm on them. As this is above Audi approved minimum the dealer isn't for changing them. However, I'm concerned the difference in tread depth may damage the quattro system in the long run.

Does anyone exactly what the story is in terms of the effect of differing depths is on the Q system? Any help appreciated as I'm getting a good deal as long as I complete before month end!

Thanks in advance