Regarding the 4GB debate, if 980Ti is out with 6GB, and is within 10-15% of 390x 4GB in performance, and if the prices are in the same ballpark, then I can see someone going with the card with higher VRAM. My point is that there will be factors to consider, instead of blindly saying "Nope, flagship card can't have 4GB". This particular reasoning may be important to some, but not all. There will be many who would go with raw performance numbers over 1440p and 4K resolutions in current games. There will be those who go for the lower priced part, even if it's 10% slower.
After all, 980 and 970 are selling right now. Many comments about 980SLI systems talk about their release eight months ago, as if all 980SLIs were sold right then, and the market had a sudden sea change where no one is buying those cards anymore. There are folks spending $700-1200 on 970SLI and 980SLI 4GB systems right now, and there will be people buying 390Xs or 390s in the coming months if they have much better performance and price ratio. When 980Ti is out, 980SLI will still be sold perhaps at lower prices.
I personally am waiting for the fastest card at 1440p resolution in the current games and in the $350-400 range. Whether it has 4 or 8GB, does not matter. I am sure, overclocking that GPU will get me within 10-15% of TitanX on those resolutions, if not closer.