The housing where the factory 5" backing plate attaches IS 5" in diameter. You can see that via this picture
Here's another view...
It would require a major major engineering design to create an adapter that would fit the 5" diameter housing to then hold a smaller, 4" backing plate.
My guess is - never going to happen.
With a gear-driven orbital - pad stall is never an issue. Instead of wishing for a smaller backing plate, learn how to go up on edge.
And edging is even easier with the new RUPES CP yellow and white foam pads. I do it all the time - I would take pictures to show what can be done with these new pads on edge but I'm usually by myself when I take neglected to respected.