Quad SLI opens up some interesting possibilities in terms of image quality. The first is the prospect of SLI anti-aliasing. With traditional two-GPU SLI, the maximum image quality mode possible for each frame was 16x AA. While one frame can be rendered using both GPUs, quad allows a single frame to be rendered by all four GPUs. This is called 4-SFR (Single Frame Render). The frame workload is dynamically distributed between the four graphics cores.
With twice the graphics horsepower it is now possible to do an astounding 32x anti-aliasing with SLI AA. This is accomplished by doing 8xS (multisampled anti-aliasing) on each GPU; the final result with all four GPUs is 32x AA output.
A second possibility is to get 16x AA with each frame. The diagram below depicts a 4x sample run through each GPU. At the end the final result is 16x AA while running in an Alternative Frame Rendering (AFR) mode.