Imaging performance slowly degrades the further you move off axis.
So it's not so much that you have to fill the 88mm image circle with a sensor, but more a case of having un-distorted stars out the edge of the current crop of sensors.
For example: the 60Mp IMX455 sits comfortably within the 88mm circle, but slight edge distortions can still be seen.
While a 4" focuser may not deliver perfection, it's a whole lot better than the 2" variety.
Expensive? Sure. But my experience is if you pay for the penny ride, that's usually what you get
