fourth pin is an output i think.
https://glkinst.com/cables/cable_pics/4_Wire_PWM_Spec.pdf
if you bypass it you don’t get a signal from the rotation speed sensor and the fan should run at 100% given 12v.
easiest solution is probably one of those 12v inverters for cars but that seems overkill. 5v fans are most likely cheaper.









does it work at all on 5v?