If you could it probably wouldn't scale very well. 3 would be more worthwhile. Take a look at this:
"Two SLI connectors are available, which would allow you to combine up to four GTX 780 cards in a Triple-SLI, multi-GPU configuration. NVIDIA limited the GTX 780 to Triple-SLI only, while Quad SLI is supported by the GTX Titan."
From this website: http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_780/3.html