My 64 buick is a 4 door hardtop sedan.
Not to me. Show me any piece of documentation, advertising, order, description, or whatever where any car company listed a hardtop sedan. A hardtop has no B-pillar, sedans do.
I really like the list that beanhead showed.
Here you go... Popular Mechanics document stating hardtop sedan. A pillarless sedan = Hardtop Sedan.
Not a car company.
The Wikipedia definition contradicts itself, as it first gives as a definition of a sedan as a car with a B pillar, then later talks about a hardtop sedan which has no B pillar.
I realize this is a discussion that has no resolution, but I still say a majority of car people would agree that B pillar = sedan and no B pillar = hardtop.