Hi Mario,
The DJI Phantom 4 Advanced is not officially supported from our side and I do not have the exact information which image ratio is applied.
From your description, I believe that the 4:3 image ratio is selected by default and cannot be changed to 3:2 even in DJI GO 4 app.
I would recommend asking the users in the following post if they have more information about the behavior.
Best,