There are multiple reports that early retail samples of the AMD RX 480 can be able to BIOS flash their reference RX 480 4GB GPUs into 8GB versions.
This is possible because the first batch of AMD’s RX 480 GPUs all uses 8GB of physical memory, even the 4GB models, so some users able to only BIOS flash their GPUs to access this additional 4GB of GDDR5 memory.
But don’t expect that not all 4GB AMD RX 480 GPUs can be flash to 8GB version, as the card need to have physically additional memory chips. Still, those who are lucky to have an extra chip on them can be potential to unlock themselves some extra VRAM and make it 8GB GPU at a greatly reduced price.
To confirm if your RX 480 4GB GPU can be unlocked or not, you need to remove your GPU cooler and read the model numbers and check them online to see whether they are 4Gbit or 8Gbit chips. If your GPU has 8Gbit VRAM chips, then you should be able to BIOS flash your GPU to access it.
Although, keep it in mind the BIOS flashing your GPU is a risky job and should not be attempted by a novice; it may result in a broken GPU.