A GTX 950 is only slightly slower than a GTX 960, I'd consider that bare minimum for Fallout 4 @ medium/1080p. And you don't actually need 4GB of VRam to run Fallout 4, I have a 2GB card that runs it fine.
maxalge does have a point. If you're paying for a i7-6700k you probably should get at least a GTX 970 otherwise you'll be very GPU limited.