It is actually much more complicated than 2-door or 4-door. Insurance company
compile actuarial tables based on the make/model of car, driver sex, age,
ticket frequency, etc. They use those to rate vehicles and drivers. If there
is a surcharge for a particular car, it is based on the claim history of all
insured vehicles of that type. Now, it turns out that certain cars are
primarily driven more "safely" than others according to the statistics, and get
the most preferred rates: things like Ford Crown Victorias and Buick
Roadmasters. Other things are driven by testosterone overloaded people more
often and get higher rates, like Camaro Z28's. There are some intersting
anomolies which I noticed while looking over a general listing of the rating
catagories published by Consumer Reports a few years ago: Porsche 911/928 are
in the "average" rating class, while 924/944 are in the very risky class.
Ferrari's are average, Pontiac Fiero's are even more risky than 924/944.
Mustang V8 GT was riskier than Mustang V8 LX. Wheeeee! The tables don't
explain why a particular car might be more risky to insure than another, the
tables simply reflect experience.