Hi,
don't have the pa 4x , I have the pa3x.
Operating system, first up, probably isn't quite as straight forward as the yammies.
Unless something has changed, you have to load user styles into the korg,vs being able to play them directly off the USB stick. That used to confuse some people that were used to the yammie system.
I
Main thing is, make sure you like the korg styles.

I personally luv the operating system of the korg, especially the style creator. Unfortunately I couldn't always find a style for some of the tunes I like to play. I'm sort of into show tunes and old standards and stuff, some of the simpler yammie styles plain suited better. So, not that bigger problem for me, I used to convert my yammies across to korg for the odd style.

Whereas the operating system shouldn't be that bigger problem, just make sure you like the korg styles.