The PSU seems to shut down when feeding it with just 2. It might be because the cables are made for just 75W and it pulls 180W through each (so it drains 360W in total).
So, I'm thinking can I plug a 3rd pci-e cable, without overclocking it just to spread the power a bit more, or it doesn't work like this?
If you have extra pci-e cable you should just plug in all 4. Having 90W on each pci-e is always better then 180W.