Check this out about 4x780ti's
http://www.youtube.com/watch?v=Aj8P2vtP2uU
You have to keep in mind if you are running 4x 760's the power consumption, drivers things and heat. But of course your question depends on what monitor set up and resolution you will be using. Once you tell us then we may be able to say but I can advise if you are going for higher resolutions forget the 760's and go 780ti on sli at least and that's it.
The Palit 780Ti jetstream will destroy everything you throw at it, even at 2500x1600.
http://www.kitguru.net/components/graphic-cards/zardon/palit-gtx-780-ti-jetstream-oc-review-1600p-ultra-hd-4k/
Can you imagine this only on SLI?