Looking at GPU usage and GPU temps, it becomes clear that at 8064x1440 the GPU's aren't even close to being engaged to their full potential. It looks like GK104 doesn't have enough VRAM bandwidth for 4GB. I guess now it makes sense why nVidia originally released them with 2GB frame buffer and 256-bit bus width. It also makes sense that the GK110 rumors say that it will come with 4GB and 512-bit bus.
Overall, right now it looks like the 4GB GK104 is a scam by AIB partners. You simply can't benefit from the extra VRAM.