The 390 with 4gb will be mostly available in asia (china).You have to find someone from around there that actually bought the card.Good luck with that.
Another thing is the memory clock on 390 is much higher than 290 so even if you get a 390 4gb bios your card may not handle the high frequencies.
Same happened with my 6950 that is unlockable to 6970 but i had to modify the new bios to the 6950 frequencies.The card cannot handle the 6970 frequencies..
Considering that Polaris is just around the corner and 4GB might be to little on a directx 12 platform in 6200x1200 resolution - I might give up on this and simply sell my good old R9 290. Vega is going to be the choice of replacement...