When AMD released the Radeon R9 390 and 390X they launches the models solely with 8GB of graphics memory. Probably a choice that they wanted to make a bigger difference compared to the 290 and 290X with 4 gigs. The extra 4 gigs can be argued whether or not it is usable, however it does add cost to the final product.
In a market where every dollar, euro (or whatever you currency is) matters it seems that soon you can purchase 4GB models of the 390 (not 390X). Opposed to your 290 you would still be looking at an incremental performance increase of, well lets say 10% here thanks to a bump in core and memory frequency. Obviously the cut down in graphics memory and thus pricing is not just to make a more budget friendly product, it is re-positioning a product against the still very popular GeForce GTX 970.
From the looks of it, three 4GB 390 models already have been spotted in Asia, from XFX and PowerColor who both have a similar design and packaging as the 8 GB versions, and then there is a new 4GB Sapphire card who plastered a new dual-fan cooler on the product as well.
Below some photos, of which the box shots clearly showing the 4GB versions. Yeah, these puppies are slowly turning into a 290 again :-)