Hi, because the engine of the Porsche 911 is slung out behind the rear wheels the sidewalls of the tyres are under a lot of stress/pressure due to weight transfer when accelerating hard. Porsche therefor approve certain tyres for use on those models and that approval rating is denoted by the 'N'. It's a case of non 'N' rated tyres being unsuitable for the Porsche so I would'nt see any problem with fitting them to a Z4. I know coming off a Porsche the tyres should be OK, but you should still check the speed rating and aspect ratio of the tyre for minimum specification compliance with your model of 'Z' before fitting.