" if you have hardware and software that can create video at >4GB/s you're pretty special."
Protip: Most game modern engines can EASILY do that with just the video card. AGP had a maximum throughput of 2166MB/s, half your requirement. That was replaced roughly a decade ago.
Bandwidth of a video card interface and the throughput of a 3D gaming engine is irrelevant to this discussion. We're talking creating video through something like Avid or Premiere/After Effects by compositing multiple streams of video and effects and then mixing that all down to a single stream for output.
Whilst some acceleration of this is done on the GPU (and this is why the 3 year old Mac Pro has 2 GPUs even in it's base configuration) the main requirement here is fast and consistent throughput to mass storage.
There are plenty of video professionals that use a Mac Pro with it's stock 256 GB internal SSD and then hook it up to an external 8- or 12-bay Thunderbolt RAID (or two) that can transfer in excess of 2 GB/sec - something like this http://www.lacie.com/as/en/pro...
I don't care how big your PC workstation case is, if you want 50 or 100 TB of storage, you're looking at external RAID or SAN anyway. The internal storage is only used for the OS, apps and scratch space.