thank you guys for your reply,
so if minimum of 3GB, then 6GB should be enough for 4k (assume SLI will be used in case if FPS was not well). and I think 4k will keep running for gaming for next 10 years.
here I have another question,
how the Game Engine make the difference ??
like todays if a GPU can give you 60FPS of 4k, the same GPU could give you 40FPS of 4k after two years (for example).
?