I have seen that #5 pin used when using some USB connector cables for front panel use. Since each side of the header is one USB connection, most front panel connection plugs come with two connections on them.
The connector has a blank spot where the #5 pin would be on the second row of pins on the header. That way the connector is only able to be plugged in one way only.
I believe the 5th pin is used to prevent a reverse polarity hookup when using those connectors.
See the image below, red arrow pointing to the blank pin spot on connector: