Because they marketed it as having 4GB of VRAM, however, only 3.5gb of that VRAM runs at full speed, the latter 512mb runs so slow that it's basically unusable.
It's not noticeable, and I doubt anyone has ever noticed it, but hey, free money.
If you look at the following screenshot, you'll see how the performance of the VRAM drops off significantly once it hits >3456MiB: