With GTX 970 you can play Hearthstone ~ 120fps on 4k@60Hz.
With GTX 960 it should be ~60-70 fps.
Hearthstone @4k uses ~ 1900mb of vram. GTX 960 has 2GB. So yes - it should be playable.
I didn't specify that I've got the GeForce GTX 960 Gigabyte WindForce 2X OC 4GB (DDR5) Edition so I'm more than fine, right?