Now now, I thought the 7970 would be more future proof than the 680, due to vram as well as GCN/console similarities.
I didn't think the 970 3.5gb fiasco was fine at all.
But realistically, more games are already using >4gb vram at 4K. With the passing of time and more titles developed focused for consoles, we'll see that become the norm (at high resolution).
4GB may remain to be enough as long as you don't run with MSAA at 4K. Generally even two top GPUs cannot play recent titles at 4K with 4x MSAA anyway so it doesn't apply for most users, only those with Quad SLI/Fire.