The shiniest cars come to those who pre-order.
Whilst I am sure that there are plenty of shiny cars in Forza Motorsport 4, none are so sweet as those that come free for pre-ordering. Five cars in total will be common to all pre-orders: 1965 Ford Mustang GT Coupe, 1997 Lexus SC300, 2011 Koenigsegg Agera, 2011 RUF RGT8 and 2011 Tesla Roadster Sport.
There also seems to be a number of pre-order bonuses exclusive to certain retailers, such as the GAME exclusive Subaru Impreza WRX STI and the Amazon exclusive Alfa Romeo Giulietta. Check your preferred retailer to see what they have to offer.