For reviewers AMD has been circulating a BIOS that would allow 8GB versions to be tested at 4 GB. However it was stated that all 4GB retail cards do physically have 4GB and not 8GB. That it seems was not true for some batches. Some users have reported purchasing a 4GB RX 480, that in fact physically holds 8GB of memory.

A user called bertieX over at Dutch website Tweakers.net reported that he purchased a 4GB PowerColor RX 480. The card has 8 memory chips and when he watched the GDDR5 ICs closely, he noticed Samsung K4G80325FBHC25 chips, these are confirmed 8Gbit. With eight of them  that does make a total of 64 Gbit and thus indeed there is 8 GB of GDDR5 memory present on the PCB.

The card however is recognized and utilizing 4GB as model, this means that the remaining 4 GB is disabled in the BIOS. In theory it would be possible and very simple to activate the remaining 4GB to 8GB with an 8GB SKU BIOS update. It is very doubtful that AMD will keep delivering 4GB models with physical 8GB on the cards, likely this was a snap decision or quantity discount to get the cards out fast. At this time it is uncertain if all 4GB cards sold in fact carry 8 GB of memory, according to more rumors the XFX cards also have 8GB present on the 4GB models.

The 4GB Radeon RX 480 costs 219 euro whereas a 8GB model will cost 269 Euro. Currently you'll be hard-pressed to spot a store selling 4 GB models as they are out of stock everywhere.
 

Early Batches of Radeon RX 480 4GB cards actually do have 8GB memory