The single 4 pin connector provides the same 12v power from the PSU as an 8 pin connector. The additional pins simply provide a better ground for the 12v current. Using the 4 pin connector in the plug side closest to the processor will be fine. If you have an overclocked quad an eight pin connector would provide a better grounded power 12v source.
Sorry but it's not gust extra ground, there is 2 x +12v too. that way it spreads power on more wires and more to VRM. It's much better to have all 8 pins connected since it was already provided by PSU.