IIRC the extra pin on a 4 pin motherboard header may report if the fan has totally stopped, like a failure line, to tell the mobo to shut the CPU off. I've got an old 4 pin fan here from a '94 vintage ...

On page 4, there is an "IDE Power" connector. It looks a lot like a 4 pin CPU fan connector, but it (obviously) provides different voltages that are appropriate for 3.5" IDE drives (5V, 12V) and the ...

When we rotate the board we can see a white 4-pin CPU power header, already indicating that overclocking really is not an option (otherwise it would have had 8-pins). Then we see two SATA 300 and two ...

The Balanced Technology Extended (BTX) interface specification was developed by Intel to provide standard interfaces and form factors in an effort to address the thermal and acoustical issues ...