You have two options:
First use y splitter cable for fans. Connect two fans on this cable and then connect its single end on that remaining 4 pin slot. 4 pin headers are compatible with 3 pin cables.
Second You can use molex connector. It's one end is for fan cable header and the other is the standard 4 pin power connector to be connected with PSU's 4 pin connector.
Thanks EasyLover, ill have to see how it goes tomorrow.