The SD4 has TWO MIDI synths in it, with two separate input connections.
The first one is identical to the SD2 in all respects, the one exception possibly being the Percussion Banks, I'm still looking into that one, will report when sorted.
The second synth is the "Presets" synth, which has 64 default sounds and also 64 user definable sounds. We can mix and match and also edit sound parameters for those last 64, such as layering, effects etc. and end up with a one button command to call them up.
The second synth then would be the equivalent of your keyboard that you play, while the first one is a GM compat with upper banks, using the exact same patch list and commands as the SD2, which would likely be hooked up to your computer, MIDI file player, or the like for accompaniment.
We can do things like access the rotary speaker from the SD4's built in controls.
"Keep listening. Never become so self-important that you can't listen to other players. Live cleanly....Do right....You can improve as a player by improving as a person. It's a duty we owe to ourselves." --John Coltrane
"You don't know what you like, you like what you know. In order to know what you like, you have to know everything." --Branford Marsalis