the motherboard(ASUS P5Q DELUXE) manual says "Do not forget connect the 8-pin EATAX 12v power plug. otherwise, the system will not boot", but i found that four of 8 holes are blocked with a black plastic cap. If it is required, why the four holes are blocked although i can remove the cap?
Everything G said is correct. Like he said it all boils down to CPU power. If you have a CPU with higher power requirements then you should use the 8-pin because it allows redundant pathways for current to flow to the chip and reduces the chance of burnt pins. The 8-pin will work with all CPU's, however the 4-pin may not work with all CPU's, especially at high overclocks. I believe the power breaking point is 70 watts. If your CPU dissipates 70 watts then you should use the 8-pin for best results. This is general advice, many people have run CPU's with 70 watts dissipated power using the 4-pin and never experience any problem.