FSX, 737, 747. All of FSX jet aircraft are all passenger, so using GSX to customize them for cargo, limits them to crew and loadmaster without any cargo operation, except for luggage.
Then don't configure them as cargo. I'm not sure I understand your question, if you want to configure a default airplane as cargo to TEST if the limited cargo crew works ( I thought this was your goal ), you sure can do that. If you don't want to do that, leave them as not cargo.
There is a GSX prompt to open door 4, but unfortunately there is no door 4 function available for FSX default aircraft.
By default, GSX should call exits on default airplanes ( 737 or A321, for example ) with names like "Exit 1", "Exit 2" or "Exit 3". NONE of them, by default, is configured to call a door "Door 4" so, it seems you changed the default configuration, if you really see a prompt for Door 4.
And, the "Name" a door is referred to by GSX is completely unrelated to the airplane door "function". What really matters is the variable associated to it, which is found in the GSX Airplane configuration editor, and can be either one of the 4 possible default variables for the doors OR a custom XML code, if the airplane has custom door controls.
The position in the diagram doesn't matter either, it's just an example, the only thing that controls the door location are its coordinates.