The PA3x was a bit piecemeal in that it had many features that were better than the PA2x (Which is still one of the best if you looking for a used arranger) but not all, (This is common with all manufactures) the PA4x brings everything together as a complete whole (Features from the PA3x and full improvements on the PA2x) hence it’s a more complete board. (Korg are also one of the few manufactures that add new features, sounds etc. via updates, (On pretty much all their boards) whereas others force you to by a complete new board)
No matter how good a board is though, (No matter many recommendations it gets) it all comes down to personal preference in the end, so try them out and pick the one you like best. (There is no right or wrong about it)
Hope this helps
Bill