One thing to bear in mind here, is the difference in the makeup of summer versus winter tires. Of course, the tread patterns are very different to account for summer driving in dry or wet conditions - versus winter driving in snow or on very slippery ice.
But its also important to remember that the rubber itself can be very different. The rubber in summer tires can get stiff and not grip very well at all, even on a completely clear and dry road, as the temperature gets close to freezing. Its one of the reasons why many BMWs have a low temperature alarm when the temperature falls to +3 degrees C / 37F. The rubber in winter tires is formulated so they don't stiffen up in the cold and still give good traction at low temperatures and on slippery roads. This also means that if you drive with winter tires in warm spring weather, they will wear out fairly quickly.
After years of driving in Canada, my vote is to garage your car over the winter. This will avoid all this trouble and the risk of damage from de-icing materials like road salt (if used in your area and which will EAT your car ALIVE), or road gravel. In my experience, there isn't much that ages a car faster than driving it in winter. Lots of dandy advice over on the E85 forum about winter storage. I wash and wax my car, top her up with gas to avoid tank rusting, pump up the tires to 40 - 45 psi to avoid flat spots, put the car on a good battery tender, toss on a good BMW cover, and pray for spring! Do this and years from now, your Z4 will still look brand new. Folks who ask me how old my car is are shocked when I tell them its a 2006. The usual answer is "don't you mean 2016?" "Noooo - 2006!"
And congrats on the Misano Blue Z4 - great color choice!!!
