If you mean 4pin "main connector" as the +4 pins on the 24 pin connector, those extra pins are optional. If you meant as a replacement for the ATX12V connector, each 'molex' connector has only one 12V line each capable of carrying about 5A max, so you'd need a molex-to-ATX12V adapter which has two molex connectors connected to separate wires directly to the PSU to safely pass the 8-10A modern CPUs require.
It will work assuming the PSU has sufficient total output on the 12V rail, but isn't recommended. I've done a similar hack to fit an old ATX PSU that didn't have an ATX12V cable into an old P4, simply cut the ATX12V from the P4's original blown-up PSU and spliced it into the other PSU's wiring.