I got to try this your procedure..I am installing 3 msi 280x and 1 HIS 280x..system works but everytime I am trying run CGMiner, got 510C temp and freeze right away.
msi first 16x to 16x
his first 1x to 16x
third msi 16x to 16x
fourth msi 16x to 16x
all powered..still can't run cgminer..I thought it was motherboard.
I use the same board with 5 gpu's type r9 290 cards.
No shortening needed for a Asrock 970 extreme 4
I use two 16 x16 riser cables on the first en second 16x slots. The are working than at x8.
The last 16x slot can only be occupied with an 1x riser cable, because an 16 to 16x cable in this last slot would block the pcie x1 slots from working.
the other x1 slots can now work with x1 risers (see manual).
Install one by one starting with the first x16 x 16 cable than the other 16 x16 cable on the next 16 slot and than a x1 cable in the last 16 slot
Always easy continue to install in this hierarchy
It some time takes a lot of energy to customise a system.It is although strange that the same type of board is not always working the same way.
Good luck