The confident and muscular new BMW Z4 is the most luxurious convertible this side of $100,000. It is undoubtedly one of the best do-overs in recent years, designed by Juliane Blasi and Nadya Arnaout of BMW's Munich design studio. The new car, now with a retractable metal roof rather than a soft top, soothes rather than strains the eye. Two obvious advantages of the folding metal top are readily observed: the cabin is virtually as quiet as any conventional hardtop, and it ends worries over the decline of a fabric top. But the aluminum panels and the top structure add roughly 200 pounds compared with a soft top; the Z4 can reach 3,500 pounds, about 200 pounds more than a Corvette convertible. And as with most retractable hardtops, this designer sandwich, once folded, leaves room for little more than a few side dishes.