Danke Shoen, could I buy an aftermarket hub adapter for my S4 splined shaft to enable the S3 steering wheel to work?nope, not if using the original AR wheel.
the S4 has a splined shaft, the S3 uses the woodruff key.
You could get an aftermarket nardi wood wheel (for ex.) to fit as then you can get a 'hub' to fit either splines or keyed shafts, as needs be.
But you can't remove the hubs on the factory AR wheels.