This is just a prototype of BMW Z4 with M sport package caught testing at Nürburgring. Changes that were made are probably the design of the body and the equipment inside the car to improve the car performances. As you can see, more exactly differences between this prototype of BMW Z4 M and BMW Z4 are the front and rear bumper, probably the length of car, the mirrors, those bigger wheels and the headlights.
At the M sport package, the buyers can request custom paintwork and exclusive wheel design (source: motorauthority). I forgot to say BMW said that BMW Z4 M will not go anymore in production; so why they were testing a prototype of BMW Z4 M at Nürburgring?
I prefer much more this BMW Z4 with M Sport Package instead of normal BMW Z4. I like more the design of BMW Z4 M car than normal BMW Z4. Hopefully BMW will change it’s mind and will put this car into production line.