According to posters on Tweakers.net, confirmed in a blog post by the site, selected 4GB RX 480 models have been found with 8GB of memory physically on-board. Previously, AMD had suggested that that 4GB/8GB split between the two models was hard rather than soft.
This is not an unheard of modification from AMD or other hardware companies. In the past the AMD 7950 was upgradable to the 7970 and selected AMD X3 Phonoms could have a 4th core unlocked.
The reason behind these possible upgrades is that it is often cheaper to produce 1 hardware configuration then disabling features via software, rather than 2 separate hardware’s configurations. Quite often the cheaper models will have minor defects that rule them out of being sold as the more expensive models. So for example only some of the old X3s could be unlocked, presumably because there was an issue with the forth core.
At the moment, for owners of the 4GB RX 480, you won’t be able to unlock the extra memory as the BIOS files for switching between the two modes are not publicly available, however I assume it is only a matter of time before the BIOS leaks, making the RX 480 an even more tempting proposition for mid-range gaming.