I have the same mobo - but you will need pci entenders - there's no way you can fit 4 double slot gpu's in this mobo without extenders.
I have no problem with the risers, as I understand it, the powered ones are from 1x -> 16x PCI-e so I plan to get four of those (one for each card)
That's not my question but it looks like he answered it in a comment anyway:
I just want to make sure if the 1x to 16x Powered Riser Cables dont diminish the hashing rate of the cards connected to them? I suppose they dont but just want to make sure
No, the 1x slots have no effect on hash rates. The full 16x bandwidth is needed if you are using the card for video games.
Read more:
http://www.coinminingrigs.com/quad-7970-mining-rig/#ixzz2W8BEoe25Thanks