That doesnt answer the question of WHY. It says that the 2x has USB ports. And doesnt say anything about the 4x lacking them. Why does one have it and the other does not? What was the reason for not including them on the 4x? Why are they necessary at all?
Read.One have a PCI "switch" for communication to GPUs on rises (The USBs) and the other one only communicate over the PCI bus.
Two different products, you choose which one you want.
