I just use a second tender, the small letter SP Lines one since the real one is a converted tender. I have never found a good SP auxiliary tender, only the UP ones on DLS and Steve Lerro's N&W ones, which are too short.
There are two Union Pacific aux tenders post 2006 available on DLS by Gillesd81.
<kuid:609568:101351> UP Auxiliary Tender 809 (Jim Adams) SR
<kuid:609568:101352> UP Auxiliary Tender 814 (Joe Jordan) SR
There are two Union Pacific aux tenders post 2006 available on DLS by Gillesd81.
<kuid:609568:101351> UP Auxiliary Tender 809 (Jim Adams) SR
<kuid:609568:101352> UP Auxiliary Tender 814 (Joe Jordan) SR
Not sure how easy it would be to repaint in Southern Pacific Lines black and white or Daylight scheme and they might well be a bit modern for the application.
Edit: Umph, as you can see I have a few auxiliary tenders (mostly K&L's fault) but none really fit with the GS-4. The best choice would be to use a second tender as Saturnr suggested. To really make it look better it might be possible to clone and repaint it to remove the black in the lower front.