Just cut the 8-pin EPS string off a dead PSU and either solder the whole thing into your PSU, or splice your 4-pin to it, plus two additional yellow +12v wires and two grounds from an unused PCIe string. 2x4 configuration is derated to 7A per pair due to concentration of heat, so 336w total. While they do sell new connectors and pins, it's probably less trouble to scavenge the parts, especially since it's only for an old 450w supply anyway.
If your board only has a 4-pin with unpopulated holes for an 8-pin, or you have already burned the 4-pin connector, then soldering the wires directly to the motherboard would provide the lowest resistance and most reliable connection, but would sure make changing the PSU later more difficult.
With low voltage wiring you don't have to worry about arcing, but must be very concerned with resistance and overheating because of how many amps it takes to get any kind of reasonable wattage. Overheating the connector anneals the pins soft to make them looser, and a poor connection is a high resistance one so gets hot. This cycle continues until the connector catches on fire.
You may think that 240w (192w if 20ga wire) should be plenty for today's 95w TDP CPUs. However as things age, particularly if the connectors have been through many mating cycles, resistance can increase to the point that the plug melts and this is much more likely to occur with a 4-pin than a less stressed 8-pin. The 8-pin is often selected by motherboard manufacturers just to reduce RMA rates. Note that just like PCIe power connectors, the CPU power plug is only rated for 50 mating cycles.
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