The Molex 4 pin power supply connector is one of the standard peripheral power connectors in computers today. The power connector itself is a Molex 8981 connector called the AMP MATE-N-LOK.
This power connector is the standard connector for all PATA based hard drives, many high end video cards, and some older optical drives and other internal devices.
Below is the pinout for the standard Molex 4 pin peripheral power connector as of Version 2.2 of the ATX Specification (PDF).
Note: If you're using this pinout table to test power supply voltages, be aware that the voltages must be within ATX specified tolerances.
You can see other ATX power supply connector pinouts in my ATX Power Supply Pinout Tables list.
Molex 4 pin Peripheral Power Connector Pinout (ATX v2.2)