actually I wanted to model these ones. But I am not quite sure how to approach a clean modeling
Your second attempt looks great. It feels really solid and heavy. Andrew Price has a good rims tutorial where he models beemer rims, so you could look at that for rim modeling advice.
In the same moment I saw his Tutorial 
But thanks for mentioning! I think with his help I should be able to make it.
I am playing around with the array modifier and the Object Offset now, I think this is the right approach, going to keep you up to date
This rim would actually be quite easy. I am in love with modeling rims. They are so much fun. If you have any questions or advice shoot me a pm.
not too bad, Not my first choice for tire tread. its too easy to come by. it is so over used. try tirerack.com and on the site search for some toyo or pirelli tires. when you find a tire you like click on it. go with some nice semi slicks. it will provide (an ill be it, small) tread view. depending on how exotic the tire is, you would ave to create your own wheel wall bump. others you can find all over the place. if you go with some pilot super sports, or pirelli corsas, I have made normals for the PSS and bumps for both. Zuzzi made a damn good pirelli roso bump. You are more likely to come across a bmw with rossos I would think. the good year eagles are only really nice because of how redily availible the bumps are.
The rims look a little thin, but other than that it looks nice, good work. I don’t know about the Z4 but I know the M6 comes with Michelin Pilot Super Sports. I can post a picture of the tread on the PSSs if you want, but its up to you 
If BMW sends out cars with michelins than I would go with PSSs. I made a PSS bump and normal map that I made availible on my corvette thread. can be found in my sig below this post. the hi res version cost 6 dollars but I made a low res version availible for free and posted it to the forum.
Yeah, thanks for that bump by the way, it looks good on my Bimmer