When the eight-pin connector and the four pin connector should be used?
It depends on what it is?
First i used 8 PIn connector but system don't start.
and i used 4 pin connector and system work.
If your computer runs on 4 out of the 8 pins then just leave it alone and have a working computer. I remember that I had this weird problem too but after waiting a few months the motherboard decided that I could then plug in the rest of the 8-pin connector.
Are you 100% certain you are using the 8-pin EPS12V connector? It looks very similar to a 8-pin PCIe power connector but on the PCIe power connector, 12V and GND pins are on opposite side from EPS12V. If you used the wrong connector, the 12V lines are being shorted to ground on the motherboard.