I always assumed that a hatchback, which in theory gives direct access to the car itself was a 5 door and a 4 door was a saloon with a boot which doesn't give access to the main interior space in the car. I could be wrong though...
Up until the late 70's you had 2 & 4 door saloons and 2 & 4 door hatchbacks. I don't know why they changed to calling hatchbacks 3 or 5 door when we all new what was going on before. By the way, my parents used to call an estate car a "shooting brake" - does anyone know why (yes,I have asked them and they don't!).
A "brake" is an old fashoined term for a wheeled carriage and the term shooting brake came about from the country sport types who used them on the hunting, shooting fishing trips. The 2,4 & 5 door arguement is correct with the assumption that a enclosed boot was not defined as an entrerance to the body of the vehicle but as we now have fold down rear seats the rules have changed....further to this some people carriers have two sets of sliding doors and a rear tailgate...does this make them 7 door vehicles?