What is the fan connector on a PC motherboard?
That connector would be called something along the lines of "4 pin cable header" where it is important to know the connector pitch. I can't quite determine pitch from that picture but common industry standards are 2.0mm, 1.27mm, 1.25mm, 1.0mm.

Its mate would be called something like "4 pin x mm socket header". You should search for board-to-wire connectors. Likely, one of these will be the one:

http://www.molex.com/molex/products/group?key=wire_to_board_connectors&amp;channel=PRODUCTS&amp;langPref=english