Early AMD RX 480 4GB retail units actually have 8GB of VRAM which is unlockable via a BIOS update, reports WCCFTech.
This is a somewhat bizarre move by AMD, as the whole point of shipping the RX 480 with 4GB of memory as opposed to 8GB is that manufacturing costs would be cheaper.
The theory goes that AMD, sticking to its word, wanted to make sure that the RX 480 was available at launch for the advertised MSRP of $199. All the while supply of the lower density memory chips for the 4GB variant were still ramping.
This meant that instead of going ahead with the launch and only having the 8GB RX 480 version available on day one, the company decided to bite the bullet and sell 8GB RX 480s with the 4GB BIOS instead.
This would mean that only these initial RX 480 units would have this flashed BIOS, but it also means that early adopters got even more of a bargain with their cards.
There is currently no working 8GB BIOS leak out at this point, so any unlocking you attempt will be at your own risk.
The $199 4GB special only applied to the US. That is probably why we only got the 8GB version, because we anyway pay in the range of a R9 390 / GTX 970 for it… The RX480 4GB should be ~R1500 less