In Most times you can use it - simply look at connectors - you 'll see that 20pin fully identical to part of 24-pin - simply connect it with right direction and position. Many motherboards works great with it. You can leave the other 4 pins untouched.
4 extra contact uses by very powerfull systems .....
If wires from connector don't takes warm-use it greatly
I also wrote about this problem in user guide of mine Pilot psu with pics of pinouts of connectors.