By default the GeForce GTX 1080 supports two GPUs in SLI – as games have evolved, it has become increasingly difficult to provide beneficial performance scaling with 3 or 4 GPUs, and as such 3-Way and 4-Way SLI configurations are no longer recommended.
Not impossible mind you, simply not recommended. To unlock 3-Way and 4-Way SLI functionality, enthusiasts will need to acquire a free "Enthusiast Key" from a NVIDIA website. Simply run the key software on your PC, follow the steps, and you’ll unlock 3-Way and 4-Way SLI functionality.
With the transition away from 3-Way and 4-Way SLI, our recommendation for enthusiasts is to now use 2 GPUs in SLI for graphics, and offload other tasks to 1 or 2 additional GPUs, maximizing performance and smoothness, and minimizing latency, in particular in VR.
Alternatively, use Multi Display Adapter (MDA) or Linked Display Adapter (LDA) modes in compatible DirectX 12 games, which can be developed to support up to 4 graphics cards.