Originally Posted by
ex-egll
Ummmm, interesting situation! As these fans are brushless it complicates the issue, I have tried a couple of tests using the multimeter and I may have a solution, no guarantee it will work for you and there is always the (very slight) possibility that the fan could be damaged. This will (may) only determine the red and black wires as a pair, but at least the yellow will have been eliminated. Once the red and black have been identified it is a 50-50 chance you will connect them correctly first time, if not just reverse them.
Put the multimeter on the 1 or 2 M resistance range and put the probes on a pair of wires (do not allow fingers to touch bare metal on the probes or wires as this will affect the reading. Eventually you should find a pair that show some resistance, albeit for a short time, these will be the red and black. It is likely that they will only show resistance in one direction so in addition to checking all the pairs, check them all again with the probes reversed if there is no resistance shown. If this fails to identify the pair then go up a level on the meter to 10 or 20M and repeat. It worked here on the 2 fans I have.