I grew a bag seed I found in a commercial batch of GG#4. It grew well, but the stems were red the entire time. I also have heard a few people say that GG#4 commonly displays red stems. I know it wasn't a deficiency that caused the red stems on my plant, so I chalk it up to genetic predisposition. One of the strains that's in the mix that makes up GG#4 has the trait and it seems to be commonly passed on.
As long as every other aspect of the plants are growing well, I wouldn't be too worried about the red stems. And be careful not to overcompensate with the fertilizers, etc. Just let it do its thing.