While I've never used the 4 pin plugs, I have use the 2 pin plugs. They seem to help the overall smoothness of the idle and the power seems to be better. Not sure about fuel economy yet since I've only had them in for several weeks.
I did use the 4 pin in a Pontiac Fiero some time ago. In that car, I didn't notice any difference while using the 4 pin plugs.