Most 4wd vehicles need the tyres to be of equal circumference or one tyre will try to turn faster than others and break the various parts of the transmission. The car will warn if it finds one wheel turning consistently at a different rate to others, hopefully before anything breaks.
This is why X3 and X5 4wd are so picky about their tyres, especially when staggered - they all need to be the same circumference which is not easy to achieve when you have different aspect ratio front to back. The star BMW use to delineate "approved" tyres (i.e. those with matching circumference) also provides a revenue opportunity
