Hi John, only thing I can think of , on a simple set up like mine would be you’d have the better feel of the Pa4x keys vs sx keys.

On a more complex set up and the brilliant midi functions on the korg, you could probably do a lot more, like using Pa4x sounds for melody, probably layering sounds.
But would you really want to basically ignore all the work you have done with your sx registrations .
Sort of glad I haven’t got mine anymore, or I’d be trying to work out all sorts of ways in combining the 2 ( which I did way back when I had my pa800 and sd2 midied) and I wouldn’t be playing again. Haha.

First option should be simple, if you just want to use Pa4x as a controller for it’s better key feel. If you want to use the sounds on both, slightly more complicated set up.