I'm going to say because FFXIV is a 32-bit program, the DirectX API might not be exposing 4GB of video memory.

Go look at the two threads posting benchmarks and there isn't a single person with a result showing more than 3GB.

The guy with the dual R9 8GB cards also shows only 3GB. The 3GB cards show up as 3051-3052MB, 2997MB, or 2992MB while the 4GB cards show up as 3072. 2GB cards show up as 1991-1992MB or 1956-1960MB.