The first owners of Radeon RX 480 graphics cards with 4 GB of GDDR5 memory have noted something interesting – it turns out that most of these cards come with enough memory chips to have 8 GB of memory on-board. The only problem that stands in front of you is how to get these memory chips working.
The solution is simple – yes, these cards do have 8 GB of on-board memory and all you need to do to get all the memory to work is to change the video card’s BIOS with one taken from a RX 480 card with 8 GB of VRAM. This is the hard part but there are some software tools such as ATIFlash that can help. Of course flashing a RX 480 4 GB card means that you will have to disassemble it to make sure you have 8 GB of memory on-board, which will void the warranty but this is the price to pay.
Here you can find more detailed information as well as links to the needed software but do this at your own risk.