You may not want to. A standard 4-pin fan works on 12 VDC, A standard USB port can supply 5 VDC. Given that voltage, the fan MAY or may not start up. Once it is running, it will run at very low speed and push much less air than you expect. For large fans, the current draw MIGHT be a problem - a USB port can only supply 0.5 A max. A small 80 mm fan may use 0.2 amps or less, so that's OK, although a heavier current is required to start the fan. At decreased voltage, probably less current is used.
If you still want to do this, the tech info is this: the 4-pin fan's Black wire on Pin #1 is Ground, its Yellow wire on Pin #2 is for +12 VDC. The other two wires are not involved in power to the fan, so leave them unconnected. From a standard...