I love the PA4X. I have found zero bugs. It was well worth the upgrade from 3X. New sound source, big color screen, direct load from USB (finally), I don't believe a 5X will appear for at least another year or so, but I don't know for sure.
The vocal processing and harmonizer are outstanding!
Korg is on a 4 year release schedule with the PA1234x series.. the PA4x is not even 2 years on the market...