\The additional 4 pins were added to support the higher power draw of newer (at the time) Core 2 Quads. The 4 pins will happily provide power to most lower end CPUs, but if you have something like an I7 I would double check the TDP and make sure you can provide enough power with the 75W that the 4-pin can provide and that you don't need the 150W that the 8-pin provides. Personally I don't think it will work and even if it does I would still upgrade the power supply so you have all the right conections given off the right amount of power.
But I was just thinking of going with a molex to 8 pin as it is cheaper. Will it still work with the adapter?