There is no difference in voltage; both the 4-pin and 8-pin versions supply 12 volts. The difference is that the 8-pin connector can supply more current, since there are twice as many pins.
For lower wattage CPUs, you should be able to get away with using only 4 of the 8 pins (the 4-pin connector from the PSU will fit into half of the 8-pin connector on the motherboard). The exception would be motherboards which don't bridge all of the 12 V pins together internally (e.g. the motherboard individually wires different 12 V pins to different phases of the VRM); in that case, you would need some sort of adapter, or a PSU with the proper 8-pin connector.
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