RX 480 4GB cards are often actually 8GB models flashed with 4GB firmware. This makes it possible to upgrade some 4GB cards to 8GB models with just a firmware re-flash. It may not work with all cards even the earlier models but it seems to be a common at least with the early RX 480 reference models. The card should have 1GB memory chips as 512MB chips are not used in 8GB model and so will not work with the 8GB firmware.
Download MemoryInfo to check if the card uses 1GB memory chips.
Memory Name should have the memory chip name that is known to be 1GB. In my case this field was blank. Alternative way to verify is to physically remove the shroud from the card and read the memory chip model written on it.
Download ATIFlash and export the current BIOS to a file for backup purposes by hitting
Save button and browsing for the location to save it. Get the appropriate firmware for the card such as the AMD RX 480 8GB Reference BIOS. Click
Load Image to browse for the downloaded BIOS file and hit
Program to flash the new bios.
Flashing process could take a while and should not be interrupted. If the process is not successful check that the firmware is correct for the graphics card. If successful reboot for the new BIOS to load.
Download TechPowerUp GPU-Z and run it to verify that the card is detected as 8GB model.
The card should not be identical to 8GB model except for the box it came in. Benchmarking to verify that the full 8GB memory is used may be done to be completely sure the card works as expected.