Yeah I am a bit dubious of SLI and all the issues, some games flat out don't have SLI profiles so you're stuck with one card. To truly max 4k you'd probably need two 980ti's or Fury x's anyway and that's just crazy expensive, especially here in the UK.
If fallout 4 ends up like skyrim you'll need about a million gigs of vram to use all the texture mods
Have you played any witcher 3? The nvidia hairworks on that game just tanks performance, like 20fps+ Drop. I hope they sort that out soon.