Generally I agree to @souha13;. The gains from 4k over 1440p should be minimal, but require tons of money to make it happen.
If you can get a really, really great deal for a gtx 970 AND a board it might be worth it, but as pointed out before it's not a very good solution for 4k either, partially because of the limited memory.
PCGamer always does nice Benchmark comparisons which you can find here:
http://cdn.mos.cms.futurecdn.net/GNnbaKWSq8Sfh4GhcDNnDf-650-80.png
(benchmarks on high presets)
There is also a youtube video with some benchmarks comparing 2x970 with one 1080:
https://www.youtube.com/watch?v=8TJZsbeQWk0
Just make sure to disable the sound to get rid of the music.
I mean in the end the question is how much you are willing to spend and how much of a performance gain you are expecting.
But on 4k I don't think you will be happy with either result (1070 / 2x970)
And while it might still be fine with Elex, you'd then look around to upgrade again for the next game.
One other thing: If you are into VR, then the new GPU generation also has a huge advantage due to additional performance gain by not rendering everything twice (once for each eye) but creating one "barebone image" which is then rendered individually for each eye (or screen in a 3 screen setup).