Hartge is a BMW tuner, kind of how Brabus is a Mercedes tuner. I am pretty sure that once you replace enough parts on a car, you are allowed to re-badge it with your company car. Its similar to what Brabus did with the old SLK. The replaced so much stuff on that car with their own in-house parts (brakes, suspension, interior trim pieces, exhaust... you name it) that they were able to sell it under their name. I guess Hartge just replaced a lot of parts on the Z4 now too. They also make a pretty sick (and fast) mini copper s.