Well, taking a look at some Firestrike and Firestrike Extreme scores, the GTX 960s actually scale pretty well and come very close to the GTX 980. In the regular Firestrike, you get a score of:
GTX 980: 11,900
GTX 960(SLI): 10,800
for a difference of about 10% in favor of the GTX 980.
Meanwhile in the Firestrike Extreme:
GTX 980: 6200
GTX 960(SLI): 6100
for a difference of 1% or so. Basically a rounding error.
However, the big difference will be in the 4k scores, where the 4GB GTX 980 walks all over the twin 2GB 960s, because adding two 960s together still leaves you with an overall average of 2GB per GPU, vs 4GB for the 980 which allows it to handle higher resolution textures better.
That being said, in real world gaming benchmarks the twin 960's actually do quite well at 1080p and 1440p, and to a lesser extent 4k depending on the game, and they'll deliver about 80-90% of the frame rates of the 980, but if you are gaming at 4k, then its REALLY game dependent. Some are good at 4k on the twin 960s, some really hammer the 960s at the same resolution.
So - if money were not a factor, and you're definitely gaming at 4k, I would go for the 980 4GB, then sell off the 960 to recoup the money. If you're NOT gaming at 4k though, I would just jump to a SLI'd 960 setup which you can get for an extra $200 or so (plus a SLI bridge).
(disclaimer - I'm running twin Gigabyte GTX 960's right now...)