Well, here goes my opinion. That no post in the middle is also known as the "B Pillar" which is used to support the rear doors.

Without it, the only door you can have are suicide doors which are hinged in the rear below the "C Pillar".

I think the reason people "think of sedans as four door cars" is because 2 door cars these days rarely have rear seats, obviously the Mustang
being an exception. Yes, a hardtop always looks better because you get rid of the "B Pillar" and the the rear doors that go along with it.

Can anybody think of a hardtop (in other words without a "B Pillar") with 4 doors? Please let me know.