Just cause that's a molex doesn't mean it's wired to the same pins.
You probably have extra signal wires and stuff you don't need.
Just take the two wires from the plug part, black and red, and connect t those to the other black and reds on the fan. Cut off the Molex obviously. Solder and heatshrink, crimp, or connector it together.
If there's not the sake colors on the fan as the plug, just try touching the wires to each one while it's plugged in until you get it to spin. It's low DC voltage so don't trip out. It won't really be a big deal. Just try not to let the plug wires touch.
But as you're checking, you'll wanna have both wires touching the fans wires to complete the circuit. You'll figure it out.