Ah, the dreaded mythical song error 4! How shall even begin to explain this unexplainable phenomenon! First of all, let me offer you some suggestions as to what to do. First of all, this error will stick with your GEM midifile, no matter what you do to edit it. Don't worry, the file will still play, just escape everytime the message comes on. In fact, you may be able to continue programming the song. There are other song errors, like song error 14 or something insane like that. It's like a little virus, that sticks with the file. THat is why when you program, you should always save whenever you add something new. Save it in a different file. I even developed a naming convention, you know, the song name with numbers showing the different versions, a higher number meaning a later version. That way, whenever the nuisance of Song Error 4 arises, you can go to the previous version of the song and continue from there, after turning off the keyboard and resetting the memory.
I used to have an SK 76. I was one of the first ones to get it actually. Great drum organ and drum sounds. I used it in one or two songs and every engineer that heard the songs would ask me what drum set I used.
Unfortunately, one of the biggest weaknesses of the keyboard is its relatively unstable sequencer. Powerful sequencer, but unstable. With the above workarounds, you can still have a great programming session with it though.
Hope this helps!