"Hardtop" can mean several things. As naukkis said, generally in American cars it means the doors do not have frames around the windows and in a 4-dr there is no window post (b-pillar) between the front and rear door. If it has window frames, it's called a Sedan. But there are many exceptions. Chevy had something everybody calls a "hardtop post". My Subaru DL had no window frames, but I never heard it called a hardtop.