The car is 100% un-drivable in the snow (540i and 540ia sport) with stock tires. You can't turn, you can't stop, and you can't go! I highly recommend Pirelli Winter ICE (as opposed to the Winter 210P). But their sizes make them difficult to find than the 210s. I also recommend 16" wheels as some of the potholes that appear in the winter don't mix well with the low profile 17" wheels. and finally, yes you MUST get all four wheels - otherwise you will wreak havoc on your DSC and ABS.