the often referred to as "extra" four pins (24 pin vs 20 pin) are PCI express video card power for the motherboard feed to the PCI express video card ?????
you can get a 20 pin to 24pin adapter which has jumper wires from the 20pin wires over to missing extra four pins.
generally speaking though a 20pin PSU is getting obsolete, like Tumbleweed has requested, please submit the PSU maker and model along with the amps per rail which is found on a sticker on the side of the PSU.
http://www.pcdoctor-guide.com/wordpress/?p=3530
The 20-pin to 24-pin ATX adapters are pretty much worthless in reality. The reason for the move from a 20-pin ATX connector to a 24-pin ATX connector is to provide more overall power. Each individual wire can only supply so many amps. These "adapters" simply split some of the wires from the 20-pin ATX connector in order to end up with the 24-pin connector. These splitted wires cannot provide the full amount of power compared to a native 24-pin ATX connector. Also any adapter that is not native to the copper wire itself from the PSU increases the electrical resistance of the electricitry coming from the PSU, through the adapter, and to the component(s) it is attached to. This increased electrical resistance can result in lower voltages compared to a direct connection from the PSU without a adapter.
You cant / shouldnt run an SLI system off a 20pin PSU & the 24 pin adapter does not make up for the lacking PSU power especially at the 12volt side of the equation. The extra 4 pins actally deliver another rail of the PSU as I understand the equation.