Now, the questions that I got for you:
Are the steps above correct?
My risers have 6-pin, Molex, and SATA power connectors at the same time to chose from. I heard connecting via 6-pin is preferable but can't recall why so.
Problem is, after I connect 4 GPU to the PSU, I won't have any free 6-pin connectors left.
So, what's the smartest of the two: to buy some 3rd party 6-pin cords and connect the risers via 6-pin slots, or to connect them via Molex/SATA official cords I received with my PSU?
either way if both handled properly by understanding its power handling of each cables then you sure safe with it, here is the guide for reference on each cables.
http://www.gpuminingresources.com/Thank you very much for the link!
However, I studied it and now I have one more question: how much power do I need to supply directly to the card via the 6-pin on its top
If you will notice on the picture, the cable with 8pin (6+2) PCIe can do as much as 300w however on my experience it does also exceed it to 350-400w, so basically 1 PCIe cable per GPU is ideal, to your query on how much power you need to supply will always depend on what type of GPU you want to install, they are different in terms of power requirements.
and how much do I have to supply to the riser (via Molex, apparently)? Or is it just the combined power supply that counts, and the proportions don't mean nothing? That's not covered on the page you gave me, at least not directly.
yes it may not directly explained, but you can also see the molex wattage so with it in riser that has 75watts via molex