As far as I know a coupe doesn't have a "post" and a sedan does.
Example: Look up a '55 Chevy 2dr Sedan and you'll see the post in between the front window and back window. Check out a 2dr Coupe and you'll see when all 4 windows are rolled down there is no post. All open. They even had a 55 Chevy 4dr Coupe, VERY rare. I've seen plenty of new JDM cars that are 4dr coupes.
To add to the confusion though think of a 32 Ford 3 Window Coupe. Just 2 door with no back window much like AZCivic's description of the late model Cadillac Coupes.
Speaking of Cadillacs, I did notice Cadillac has an EV SUT. I doubt you've heard much about it and since it's full EV it takes a HUGE horsepower decrease. It can be seen
here.