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/ If the last option from the previous question is the smartest, then use Molex or SATA? Why?
you can use those, just understand the power rating they can handle, basically don't use more than 2 sata or molex and it sure burn the cable. see the guide above link
The wiring in my apartment is of decent quality and each socket easily handles 15+ kW of equipment. BUT! There is no grounding line in my house, so the sockets don't have one either. Is that critically dangerous?
it has some risk, grounding protects your devices, but you can run them without it actually.