It seems like there are two limits: PCIe bus up to about 3.2 Gbps, and total network bandwidth about 3 Gbps. So the total net bandwidth limits the 4x card, and

It seems like there are two limits: PCIe bus up to about 3.2 Gbps, and total network bandwidth about 3 Gbps. So the total net bandwidth limits the 4x card, and also limits any combination of card interfaces and the built in interface (I tested many combos). Overclocking can get the total net throughput to 3.4 Gbps.