Each make and model carries it own rating for insurance. In some cases four (4) door cars are less expensive to insure than two (2) door cars. If you are comparing the same year, make and model than this could be true. If you are comparing different makes and models than it might not be the case. Other factors such as engine size, safety features and cost to repair are factors.
If the four door car is a family sedan and the two door car is a sports car then the 4 door car would be cheaper due to its vehicle classification. If the 4 door car though was an expensive vehicle like the Porsche four door passenger car while the 2 door car was a Ford Escort then the 2 door car would be cheaper. So there are variables that determine if a 2 or 4 door car will cost more to insure.
You can see how different makes and models effect the rates by getting a free insurance quote here.