I'd say for winter tyre users, consider this, the tyre compound will harden up after 5 or 6 years, so if your expected usage over that period is a lot less than what these tyres will give in miles over their life time in wearing down to 4mm, then use them for a bit longer each season. I changed my wife's old Polo after using the winter tyres for 24K over 5 years, and the tread depths were all 6mm (wheels rotated), which surprised me a bit (Michelin Alpins)