Earlier, Expreview.com managed to get a few pictures and listings which included 4GB version of the Radeon R9 390 from AMD AIB partners like Sapphire, Powercolor and XFX. The standard 8GB version of the Radeon R9 390 is currently priced at US $329 while the 4GB version would have hit the market at US $299 or even lower, putting a lot more pressure on Nvidia's GTX 970. Due to higher clocks, the R9 390 would end up faster than the earlier available Radeon R9 290 with 4GB VRAM.
Unfortunately, in case you want a 4GB version of the Radeon R9 390, you will have to look for it on Chinese market.