BMW will unveil a coupe version of its Z4 Roadster at next month^s Frankfurt auto show. The car will be officially a concept, but BMW will hurry it into production as a rival to the new Porsche Cayman S, a coupe version of the Boxster. The Z4 Coupe will be powered by BMW^s 255-horsepower, 3.0-liter, inline six-cylinder gasoline engine. There also will be an M version with a 3.2-liter inline six from the M3, upgraded to 360 hp.