PCWorks: 24 VS 20 Pins connector in SMPS(PSU)

Some motherboards comes with 20 Pins power connector and
some comes with 24
pins. What are the differences ?
I notice that Motherboards with 24 pins can also work with a
20 pins SMPS.
Will this harm the motherboards in the long run ?
What are the extra 4 pins power used for  ?
Donna
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