When you SLI or CF you only have as much VRAM as the smallest card in the setup since data is identical on the VRAM on both cards. The 4GB 760 is actually meant for an SLI setup as that is the only scenario where its 4GB of VRAM will be made use of, otherwise it doesn't have enough power to get decent frame rates at a resolution and AA setting that would need that much RAM.
TLDR - if you SLI a 4GB and 2GB card you end up with 2GB in SLI(silly), if you SLI a 4GB and a 4GB card you end up with 4GB in SLI(good)