Hi, GPU on risers would have the following connections
#1 - Power to the riser, commonly SATA interface to Molex port on the riser card, or directly SATA interface to the riser card (unsure which one you have bought)
#2 - Power to the GPU, most GPU requires single 8 pin power, or other combination is 8 pin + 6 pin or 8 pin + 8 pin (dual 8 pin)
RM850 as you pointed, have 5 connection for 8 pin power, 1x for PSU and 4x for PCIE (GPU) - This cables should come with your RM850. If you lost it Corsair is selling it separately as well.
Btw, each 580 GPU draw 185W (4 is 740W) + system power (say 100W) so total is 840W, your PSU is just not enough.