Lloyd, I cannot comment on the Pa4X as I do not own one, but I can comment on the OS ... when I switched from technics to KORG (Pa600) the OS was completely foreign to me, as technics were the only KBs I owned till then ... after about a week or two, with help from the 'KORGIES' on this forum - Donny was one of them at the time - I was quite comfortable with the OS ... I now play a Pa900 and am still amazed at some of the things you can do while playing a song - even during a 'live' performance ...
Of course, one should ALWAYS do a 'hands on' before buying a KB, but I don't think you should let the OS scare you away ...

I played a wedding yesterday, and for some of the requests I had to be a DJ and use some mp3's ... while playing an mp3, I got a request for a 'Hully Gully' which I wanted to play 'live' ... I pressed the SEARCH key, typed 'hul' and there was the location of the style ...when the mp3 ended, I pressed STYLE PLAY and began the HULLY GULLY ...
Now, I am not saying you can't do this on other boards, but I was quite pleased with how seamless it was ...

Best of luck with whatever you chose ...