I think, SD4 have more 1 GM bank Sound, not better sounds, just more sounds.

I'm accordionist, and the SD4 is simply ready and more easy to work with an Accordion, or Master Keyboard. You can use a midi sequencer & a master keyboard at the same time. (but... atention the polifony, 64notes...)

In SD4 I can make "Programs" and save it.
I can asign a diferent Preset/Program for 3 midi channel's.