The 390x will be available in an 8GB version too because of the stacked HBM memory.
Actually from what I read, the new AMD model will not be called 390x (the new 390 will be a re-brand, yay AMD?), but be called AMD Radeon Fury or something, and because of HBM memory, only come in 4GB, since the HBM tech isn't very mature yet to support more memory easily (yay AMD again?)
And I agree, if these rumors hold true and its truely only a 4GB card, then AMD might as well close its doors.
In any case, AMD isn't very interesting.
AMD doesn't innovate enough, they just sit on their architecture for years, slightly increasing core count and clocks. HBM is the first innovative thing they have gone with for years, and if you're being honest, NVIDIA has shown a working Pascal GPU module with HBM last year already - of course they aren't selling that yet, because NVIDIA is smart enough to wait for HBM2.
I'm also not convinced HBM will be this big a performance factor as AMD wants you to believe.
On top of that, I despise their re-branding strategy. Every generation they only produce one new GPU at the high-end, and you have to do careful research not to buy a 5 year old chip which was re-branded 2 times into the current generation already (some 3xx cards will be 7000-series chips <.<)
NVIDIA rebrands as well of course, but usually only in the very low-end, and those cards aren't very interesting anyway.