2x 970s are generally cheaper than 1x 980ti overall.
In games with good sli optimization 970sli outperforms 1x 980ti.
That's the good. Now the bad.
4x 970 sli is more expensive than 2x 980ti overall.
In every other game than what has good sli optimization the 980ti outperforms 970 sli.
It will take years before DX11 is finally obsolete, DX 12, 12.1 games are few and backwards optimization to bring DX11 games upto DX12.1 is extremely slow to almost non existence. Meaning it'll be BF5 or 6 before DX12.1 is fully integrated. So to play BF4, Witcher 3, GTA: 5 and any other current game, now, your best option is a 980ti.
Don't count on DX12.1 as a solution to current sli restrictions unless the only games you have any interest in...