I'm assembling a PC for a friend who doesn't know what hardware is. I just received the motherboard (MSI B450M-A PRO MAX) and I chose the Ryzen 3 3200G processor for good performance at good price. I see the motherboard has 4 CPU power pins, but I'm using a Corsair CX450 power supply with a 2x4 onboard CPU power connector.
My question is : Will it work ?
Yes.
Page 8 of the motherboard manual (linked above) shows CPU_PWR1 needs a 2x2 connection for CPU power. Your chosen power supply (also linked above) states on its page 5 in paragraph 4B: "If your motherboard has a four-pin socket, detach the four-pin from the eight-pin cable, and then plug this four-pin cable directly to your motherboard."
This is very common, and not at all unusual.
The EPS12V processor power connector is intended to provide additional power specifically for use by the processor, as opposed to simply using the primary 20+4-pin ATX power connector. This connector can have 4 or 8 pins, with the first 4 pins of the 8-pin version being identical to the 4 pins in the 4-pin version. As such, power supply cables for EPS12V are often configured as a split 4+4-pin connector that can be used together or separately as required by the motherboard.
The more power the board is designed to handle, the more likely you'll need the 8-pin connector, and some high-end motherboards may even have multiple EPS12V connectors. My system has an 8-pin connector and an additional 4-pin connector, though the motherboard manual states that connecting only the 8-pin connector is mandatory; the extra 4-pin connector is optional and is there to help supply more power for overclocking.
4- and 8-pin EPS12V connectors on the ASUS ROG Crosshair VI Extreme motherboard. Notice how the 4-pin connector on the board is connected to one half of an 8-pin cable; the other half is left disconnected.
In your case, plug one half of the 4+4-pin EPS12V connector from the power supply into the motherboard (only one of the two halves will fit). The other half of the connector can simply be left disconnected.
That connector is called EPS, which come with multiple versions, including:
The 8-pin connector is to provide more power for the CPU in overclocking situation. You can safely split it to 2x 4-pin connector, because it is designed to do so. Only 1 of them will fit the connector on the motherboard. With a cutter knife, you can also make the other one fit, too.
Fun fact: you can also use the 4-pin connector on 8-pin receptive plug on the motherboard. It works on all consumer boards I've dealing with. Server boards usually require you to fill its 8-pin connector, even if you use a 35W TDP chip. You can easily trick it to work with a cable adapter.
