WildCard999 :
Although im sure the 980s quad sli will be faster I'd go Titan Z SLI since you won't be restricted to 4gb of VRAM and will allow gameplay at much higher resolutions (and mods) to run smoother. Or even the Radeon R9 295x2 8gb in crossfire.
A quick warning on 4way and quad sli systems:
Expect stuttering
Expect lag
Don't expect every new driver to fix everything (it could make it worse)
3 way i suggest would be the max for SLI, since there are many issue with 4 way sli.
3 GTX 980s would be a good choice.