Ahh, the GS4. I had one, returned it though because of very average performance. Make sure you give it a bit of juice before deciding it doesn't work, because only one axle is actually powered and the rest are driven by the rods. If it hasn't been taken care of then it'll be very stiff.
As for the wiring, I'd be careful. The wiring in mine was quite complex and the engine is an absolute mongrel to disassemble compared to a simple split-shell diesel. I would lift off the tender and check the decoder there. If it's a Digitrax with a fat little round speaker then you might just have one of the "Kobo Custom" installs - basically GS4s that Kato USA would send to Japan for a sound installation. They have two separate decoders, the Digitrax sound one in the tender and a
motor-only decoder in the engine itself.
As for value, it's probably not worth much unfortunately. Granted Kato don't do a Kobo Custom of this anymore, but the install is not ideal and wouldn't be hard to improve upon. There's also the fact that Kato
still sell these brand new. If it's the all black "Wartime" paint one then you might get a better price as they don't make that model any more.