As I wrote a couple of posts ago, the review samples of the AMD Radeon RX 480 all feature 8GB GDDR5 memory and the reviewers got a custom BIOS that can switch the cards to 4GB mode. Where it gets really interesting is that all RX 480 cards that are shipping right now reportedly feature 8GB of memory because AMD had trouble sourcing enough memory.
Not enough memory supply
PC Perspective broke the news and now Legit Reviews backs it up with more details. AMD had trouble sourcing enough GDDR5 memory with a frequency of 8Gbps so they decided to equip all launch cards with 8GB of 8Gbps GDDR5 memory. If I'm interpreting this correctly, AMD couldn't secure a large enough supply of 0.5GB GDDR5 memory chips with a frequency of 8Gbps, hence all cards today feature 1GB memory chips. This means that if you can find a 4GB card today, and they really seem rare, there's a very good chance you'll be able to mod it into a 8GB model. At the moment, nobody has dared to upload the custom BIOS but it's probably only a matter of time until it leaks or someone creates it himself.
RX 480 custom cards to have 7Gbps to 8Gbps memory chips
The article also mentions that due to the limited availability of 8Gbps memory, AMD decided to drop the Radeon RX 480 memory specification to "at least 7Gbps". The reference cards feature 8Gbps memory chips but AMD's add-in board (AIB) partners will be free to use the cheaper 7Gbps memory chips. Legit Reviews claims a fair number of custom cards will use the 7Gbps variety as it's more readily available.