The fans are indeed connected to the bottom row pins; GPIO pin 4 and 6. (I used the pin layout scheme on Ground at Raspberry Pi GPIO Pinout )
I have not connected the 4th pin from the right, the fans only have two cables (power and ground).
Maybe as for supplemental information, the site where I bought the fan-case also provided a tutorial video. Attached with this reply is a screenshot of that video 
