No sweeping statements GM, just fact based on real world testing using maximum settings, four gpus and a 4K monitor. I
said it's not enough if you like to use maximum settings and 3-4 graphics cards. Only in a couple of rare
exceptions (Shadow Of Mordor/BF4 Mantle) could you use more than 4GB at 4k and possibly keep playable fps whilst using 1-2 gpu's.
This is correct. When you have four gpu's video memory becomes a concern because you have the physical gpu grunt to drive higher presets and aa settings which in turn consumes more memory. When using one or two cards, this is much less of a problem as the grunt is not available.