I'm a bit confused by the model you've got - there is talk in the thread about 6 X 9, but isn't the Super Ikonta IV a 6 X 6 with a 75mm Tessar lens???
I have several Zeiss Ikon folders, but not that particular model. My Ikonta is a 'B', which is simpler.
I am also sceptical about the 'bellows suction' idea. I have read it too - but I've never been able to demonstrate it in practice. As I type I have a Zeiss Nettax on the desk in front of me. It is similar to the Ikonta IV, I think, with a meter but a simpler Novar lens and no range finder. The film frame has rails top and bottom to support the edges of the film - and rollers at the sides. The vertical edges of the frame are recessed by a good millimetre and there is also at least a millimetre gap between these and the rollers. Consequently, there is plenty of room for air to pass - no way could the film form an air seal. So, I don't believe it - maybe on a cheaper badly designed camera - not on a Zeiss Ikon folder.
I am usually very happy with the sharpness of the pictures from these cameras. Even the simple Novar lens can turn in a reasonably sharp picture provided you aren't expecting a 30" blow up from a hand held grab shot.
My own experience is that people often dismantle the front element to clean it and don't check the infinity focus when they refit it.
Rangefinders often need adjustment (I don't have any folder with a coupled rangefinder, but several uncoupled and 'accessory' types).
If I were you, the very first thing I would do is get (or make) a piece of ground glass that fits across the film gate, resting on the film supports at the top and bottom. I made a small glass square for this purpose and ground the surface, it is a very useful tool. Took me about 15 minutes to make.
Place the camera on a tripod and in the bright sunlight focus on an object at a measured 10 feet away at full aperture, holding the ground glass in place under a dark-cloth. A loup of magnifying lens helps. Does the lens scale show exactly 10 feet? Secondly, does the rangefinder agree with the lens scale and with your visual check on the ground glass? You then need to repeat this for infinity.
Many of the cameras of this type that have come into my possession (I have about twenty 6 X 6 folders) have failed this test - but fixing them is usually very simple, often 5 minutes with a jewellers screwdriver is all you need to reset the focus. A coupled rangefinder would be more difficult, but first step would be to determine if this is the problem.