Are any of the other slots filled? Is there anything plugged into PCIEX8_2?
Of course as you have all 4 16x slots filled there is on way you could, sorry. But it does appear that some lanes are being used someplace? Do you have any other devices connected? USB3, eSATA, etc... Try with just the 4 cards no other devices attached.
(Note 2) For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIEX16_1 and PCIEX16_3 slots. (Note 3) Each PCIEX8 slot shares bandwidth with the corresponding PCIEX16 slot (PCIEX8_1 with PCIEX16_1, PCIEX8_2 with PCIEX16_2, and PCIEX8_3 with PCIEX16_3). When a PCIEX8 slot is populated, its corresponding PCIEX16 slot will operate at up to x8 mode.