I am using the included cabels marked CPU from Seasonic
Ok.. I think you just don't know how those connectors work and are making assumptions.
But I think we were all a little scared when we built our first PC.
The "D" shaped pin is actually made to fit into a "D" shaped connection or a "square" connection. The "square" pin only fits into a "square" hole, so that is your "key" to prevent you from doing something like sticking a PCIe power connector into a motherboard.
The CPU cable is labeled CPU because it plugs into the motherboard to provide power to the CPU. All ATX power supplies made today have them and they're all keyed the same way. It's not like you got some weird CPU power connector that goes into a different kind of motherboard that's not yours.
The reason the 4+4 looks different then an 8 is because they have to make the connector capable of splitting and working in EITHER a 4-pin or an 8-pin. This requires making two of the "square" pins of the 8-pin a ""D" shape so it can fit in either.
Just put the two four-pin connectors so the "clip" meets in the middle to form an 8-pin and gently slide it into the motherboard's CPU power header. Done.