Everything about the 4X is better than the Microarranger, except for the fact that you can carry the KMA in a holster on your belt if you wish.
Better sound engine, better styles, better operating system, better keys...
4X has the best vocal processor/harmonizer there is, short of really expensive third party ones.
Version two OS gave us a huge update, and now Version three is just days away, and can only make it better still.
In my opinion it is the best arranger available for MY needs.