Best to ask my question with an example: The processor power connector on my motherboard has 4 pins. If I get a power supply with a 4+4 pin processor connector, what happens to the unused 4-pin connector? Do I cover it with electrical tape? I mean, here's this wire with one end plugged into a power supply and the other end roaming free, just waiting to cause a short circuit if it comes into contact with something conductive.
The contacts on PSU power connectors are recessed in a plastic casing, the metal inside cannot make contact with anything outside unless you shove something in there, like the contacts of the connector its supposed to go in, or something bad like a fork. You can leave the other 4 pins roaming free, it doesnt have the ability to make a short with anything conductive by accident.