This PCIe expansion card has 1 USB controller, so in theory, HD720@15 should work for 4 cameras. We recommend not starting them all at the same time though, adding a few seconds of delay may reduce detection issues.
However, HD1080@15 is twice the bandwidth of 720@15. So yes, I would recommend adding another PCIe expansion card similar to this one and it should work (with 2 cameras per expansion card). You should also make sure to power the PCIe card as this can also cause issues (through the SATA 15-pin connector on the PCB)