HI, I can't help you since I don't play a SP but an Artis. But my experience with the combination Artis and Soundtower editor is a pretty much bad one. It is actually always a matter of few minutes until the work becomes unstable or I'm not allowed to select some parameter in the editor or I can't save the edited program/multi or I don't hear any changes in the effects or the aux effect wet/dry ratio, no matter what value I dial in, or the instrument simply freezes... the list goes on.
I'm using the editor now just to adjust and edit the single programs. The multitimbral setups (called multis on the Artis) I program on the keyboard.

Some honest words: The soundtower editor was simply crap on the PC3. It has not changed over the years when people gave feedback, while the PC3 was still the most recent top-instrument.
Experienced users know that and don´t estimate a working editor for their instrument. All is done on the PC3 itself. To some degree you can edit something, save those steps ON THE SYNTH itself and log off from the editor before it has the chance to mess with your work.
I guess it´s still the same with the newer models. PC3 users did not care at some point, since everyone with programming skills works on the machine itself. I understand it´s a problem on instruments with limited editing options. I just can advice you to do your work, save the changes physically at the hardware (if you can - I don´t know about the SP models) and log off from the editor. If you want to go on, open it anew and let it read the memory from your instrument again. It´s the only way I can imagine to work with that piece of ...
Yeah, I am pissed. Because this is something that is definitely NOT right.

any more thoughts about the Soundtower Editor with the SP4 series ????

I cannot decide whether to try it or not. I wouldn't expect too much from it, but some basic ability to add effects, EQ and volume/filter envelope changes onto basic programs would be really useful.

THanks.