Originally ATX mobos were 20 pin then they had some 20 + 4 and sometimes they had a single 24 pin as you psu has.
I have heard of people using the 20 pin and the seperate 4 pin side by side to make the 24 pin but I dont know how they get around the lugs that stop them being connected incorrectly, maybe they just slice the lug off ?
Presumably you could chop the extra 4 off, if your mobo doesnt need them and modify the connector but I would like some verification that this was safe before I suggested you did it :?