I don't know ALL the details, but there simply is no feasible way to put in two posts (B pillars) for a second set of doors that would swing towards the front. Think about it for a little bit, grab a pic of a 3 series sedan and meditate on what it would look like without a roof. You'd have to find some way to hide the B pillars (or put in some sort of rollbar). Plus, the more door members you have the less rigid the sides of your car can be. The E46 chassis was designed with high rigidity in mind, so, that would go against philosophy too. As far as I know, there HAVE indeed been made some 4 door convertibles (I don't remember who made them), but they all had a door setup similar to the Saturn 3-door concept, where each side had a large forward-swinging door and a smaller, rear-swinging sub-door. Hope this clarifies things.