CPUs used to be powered by the 20 pin main power cable. Almost all current motherboards power their CPU with a 12 volt CPU power cable. There are two kinds: the 4 pin 12V cable and the 8 pin 12V cable. The 4 pin cable is often called a P4 cable (although it's a very bad name) and the 8 pin cable is called an EPS12V cable. You must plug a cable into this motherboard connector or the CPU will receive no power. The compatibility rules between 4 and 8 pin connectors have a lot in common with the rules for 20 and 24 pin connectors. The best choice is to plug a 4 pin cable into a 4 pin connector, an 8 pin cable into an 8 pin connector, or a 4+4 cable into either kind of connector. You can get adapters which will convert various kinds of power supply cables into both 4 pin and 8 pin 12V cables. You can plug a 4 pin 12V cable into an 8 pin motherboard and it will sometimes work properly but other times it won't work at all or will burn/melt the connector. As with plugging a 24 pin cable into a 20 pin motherboard connector, you can plug an 8 pin cable into a 4 pin connector and leave 4 pins hanging over the end but you can also have the same problems with things preventing it from fitting. It will work fine if it fits. If you're buying a new power supply then the safest choice for the long haul is to get one with a 4+4 power cable because it's compatible with both 4 and 8 pin connectors.