Is a Corsair HX750 enough for EVGA 970 GTX SC SLI in 4k gaming?

Yes.
Each card uses roughly 160W I believe and the Intel system sans card won't use much more than 100W. That's 420W total, and even if I say 500W that's still fine.

4K gaming:
It actually makes little sense to game at 4K resolution. It's not going to look different from 2560x1440 and you'd get half the frame rate. Thus, if you had a 4K monitor don't use 3840x2160 unless it's a game that can do that and maintain 60FPS at maximum quality. Otherwise, just 2560x1440 for most games.

I also wouldn't pay extra for a 4K monitor. You can get a nice 27" 1440p IPS panel from Asus for about $500USD. Arguably not a "gaming" monitor due to the higher refresh than a good TN panel (so a bit more ghosting but better viewing angles).

G-Sync is incredible technology. While the Asus ROG SWIFT is really incredible it's expensive ($800USD) and as good as it is I'm used to a great IPS panel so I'm holding off for G-Sync on a better panel but that's as good as it gets really for a gaming monitor.