8 and 4 power pins are both for CPU. Use 8-pin it's better. (actually don't know why there's 4-pin on this board when you can plug 4-pin cable into 8-pin slot but anyway)
I'm pretty sure the 4Pin is there to stabalize and add additional power if needed for Overclocks and the over all power usage of the motherboard for say a fully loaded board with all it's connections maxed out. In the manual it states it recommends both 4 and 8pins to be plugged in or your system will not boot, but I think that's more to make sure all your bases are covered. I would just plug it in.