I'm pretty sure the black and yellow are the power and ground wires...you can double check by noting which wires match up to the power wires on the 3 to 4 pin adapter. The other two wire are probably for RPM monitoring (not quite sure why they came up with the 4 wire CPU fan headers and what the green and blue wires are used for exacty).
Connect the original 4 pin fan connector to the CPU fan header and make note of where each wire is. If you don't mind doing a little wire modding you could cut the green and blue wires and then get a 2 pin connector (if you have any available) and splice the green and blue wires to the appropriate wires coming out of the connector...just be sure those two wires will be connected to the right pins on the CPU fan header.
Of course do this at your own risk. I've done similar in the past but never with a 4 wire CPU fan connector.
Cricket