I had one of these delivered today. When I saw one side had three cables I thought there was a mistake. Anyway I came to the item page to see if I had made a mistaken order. Since there was no item description, I scrolled down to feedback and saw you review first.
I was pleased to see that you made the point of four cables to one side and three to the other. Nicely done. I need not worry now. (I was panicking that one fan coming off it would be default to maximum.)
However I don't understand how you mean the fourth pin checks temperature from a fan. Your temperatures are monitored within the CPU, or a temp sensor on your motherboard for case fan. (That's most likely in the Northbridge chipset.)
The appropriate signal is sent to the CPU fan header, and the fan plugged in adjusts to the signal it is given. There is no measuring temperature going the other way. Maybe it's to return fan speed data, as CPU fan speed is monitored without a doubt.