Two Convertibles with some similarities, but otherwise different in most aspects such as the height, starting price, horsepower, fuel economy, ground shadow size, and maximum occupancy. First of all, there is a $14,250 difference in the base MSRP. The 2018 Alfa Romeo 4C Spider has a higher starting price than the 2018 BMW 4 Series Convertible. Also, the 2018 4 Series Convertible has bigger and thirstier engine; the 2018 4C Spider gets 3 more mpg. In addition, not only 4C Spider is slightly smaller, but it's also significantly shorter than the 4 Series Convertible. And finally, you'll seat two more passengers in the BMW 4 Series Convertible than in the Alfa Romeo 4C Spider. Among similarities are the body structure and market segment.