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 ...
I just bought an ASUS M4A785-M AMD 785G Socket AM3 motherboard to replace a bad board in a free HP I got. I transfered the CPU, heatsink and fan onto the new board and noticed that the fan connector ...