BMW's best selling 3-Series has for nearly four decades been the brand's introductory model – the first taste of a BMW for prospective aficionados. As the BMW range has grown, the larger 5-Series and 7-Series have found more sporty, more stylish and more upmarket coupe derivatives in the 6-Series and 8-Series respectively. Now, the pattern is set to continue with the 4-series, to be shown as a concept at the NAIAS (Detroit Motor Show) in January, but one already slated for production within 12 months