What we know now, based on review evidence:
1 GPU is not enough to even run 4K at semi-max settings.
2 GPUs are not enough to run 4K at maxed settings with 4x MSAA where vram matters.
This isn't even maxed, it's using post AA and not MSAA, vram doesn't matter in Witcher 3 as well, its all GPU processing power.
4 GPUs would be required to truly max and have 4x MSAA.
What's the major logical assumption moving forward?
Future games will be more GPU demanding, not less.
So in the hypothetical scenario, 2 years from now, applying current-gen Fury X/980Ti scenario:
1 GPU is not enough to run 4K at medium/high.
2 GPU is not enough to run 4K at semi-max settings without MSAA.
4 GPU is probably enough to run 4K at maxed without MSAA.
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I used to believe that 4GB vram was not enough at 4K but certainly after reading Russian's posts and the evidence presented, it's very hard to argue that case anymore. Games will increasingly demand more GPU grunt and texture quality is already 4K for some, 4GB will be enough until games start pushing 8K textures, at which point, these gen GPUs will just tank due to lack of performance in general, so it doesn't matter.