It’s another fine day on the test track as a BMW roadster–with some groovy camouflage–zips by.

But take a closer look at the photos and you see that this roadster–a redesign for the Z4 that’s due out next year as a 2010 model–has a folding hardtop.

Sure, the top on this prototype looks like it is made of fabric, but closer inspection shows that it looks like fabric has been put over the solid roof panels–note the cut line in the roof, near the rear window.

Peering past the psychedelic coverings, the new Z4 is gaining a more muscular look and grows about 1 inch in overall length and width. The roadster runs on the same chassis as the recently launched 3 series.

BMW plans to fit the new Z4 with more-powerful engines, including talk of a twin-turbocharged 3.0-liter inline six, and a diesel. Fans of the M performance range might even see the M3’s 4.0-liter V8 slipped under the new Z4’s hood. The engines will also likely be fitted with stop/start technology to boost fuel economy and reduce emissions.