SamSerious :
I wouldnt buy a 4GB graphics card for 250$ and more in 2017, its just not future proof anymore.
I'm not sure any card at the performance level of an RX 580 or GTX 1060 could be considered all that "future proof" though. These cards are best suited to 1080p, and at that resolution 4GB is currently plenty. By the time games really start benefiting from having more than 4GB of VRAM at 1080p, the card's graphics processor probably won't be able to run those games at high settings with stable frame rates anyway. And at 1440p, these cards are already hitting the limits of what their graphics processor can do. 1440p typically only requires half a GB or less additional VRAM compared to 1080p, but the GPU has to push 78% more pixels per second, making the card's raw performance more of a concern in that case.
I would say that 4GB is still a reasonable amount of VRAM for a card in this performance range, and additional memory might not see much use unless you keep using the card until the point where you are getting unstable frame rates at medium settings at 1080p. Moving up to a card with 6 or 8GB could still be considered reasonable if the price difference isn't too huge, but these cards are already marked up higher than what they launched for, stretching people's budgets as it is. The GTX 1060 does offer 6GB for around the same price as a 4GB RX 580 now, though if "future proofing" is a major concern, the 580's superior performance in most DX12 and Vulcan titles is probably worth considering as well.
I would be more concerned about the 3GB version of the GTX 1060. With 25% less memory, it's going to be running into performance issues sooner than a card with 4GB, and having 10% of its cores disabled isn't going to help.