if the 4GB on the 390 is true, then it would be prudent to wait for 8GB version of it I think.
Unless HBM is really magical, that 4GB is still going to bottleneck the card itself, especially the way games are going Remember the 970 fiasco? if 3.5GB has issues, I don't see how 4GB is going to last much longer.
Also with Anandtech's article, I had always assumed that OEM cards are going to be the same as actual consumer grade cards to avoid confusion, meaning that if OEM 380 is 285 rebrand, 380x might also be a rebrand.
I will still keep watching, but I'd be more interested in 8GB 390x, not the vanilla 390x, if the 4GB VRAM turns out to be true.