OK, too much dangerous half knowledge has accumulated here... 
It would have been smart to buy 1070s with only one PCIe 6 Pin like ASUS Strix O8G...but its too late for that...maybe if you plan to buy more look out for that. For now you will have to plug in a 6 pin and a 8 pin to your cards if they have both plugs.
The power draw of GPUs works the following way: The GPU takes up all the power it can/needs from the directly connected power connectors and if it still needs more it will draw power from the riser. The 6 (75W) & 8(150W) pin connectors from your PSU provide enough power, i would plug those into your GPUs directly. 4x1070 has a TDP of 600W, check your PSU if it can deliver that much on that cable arm (sticker on the PSU has a chart which cable arm has how much load max).
As you said, now you dont have any powercables left for your 6 pin powered risers. Here you can use those 2 molex -> 1 x 6 pin adapters (1 molex = 55W for 5V and 130W for 12V) as it is unlikely that the card will pull much power from the riser when it already has excess power supply directly.
Hope this helps clear up things
(this kind of plugs im talking about; https://www.amazon.com/TechIntheBox-Express-Molex-Power-Adapter/dp/B01M2VTRTM)
It would have been smart to buy 1070s with only one PCIe 6 Pin like ASUS Strix O8G...but its too late for that...maybe if you plan to buy more look out for that. For now you will have to plug in a 6 pin and a 8 pin to your cards if they have both plugs.
The power draw of GPUs works the following way: The GPU takes up all the power it can/needs from the directly connected power connectors and if it still needs more it will draw power from the riser. The 6 (75W) & 8(150W) pin connectors from your PSU provide enough power, i would plug those into your GPUs directly. 4x1070 has a TDP of 600W, check your PSU if it can deliver that much on that cable arm (sticker on the PSU has a chart which cable arm has how much load max).
As you said, now you dont have any powercables left for your 6 pin powered risers. Here you can use those 2 molex -> 1 x 6 pin adapters (1 molex = 55W for 5V and 130W for 12V) as it is unlikely that the card will pull much power from the riser when it already has excess power supply directly.
Hope this helps clear up things
(this kind of plugs im talking about; https://www.amazon.com/TechIntheBox-Express-Molex-Power-Adapter/dp/B01M2VTRTM)