I have a 12V 0.8A Nidec TA450DC fan in some equipment and I need to slow the
fan down to about 75% of its speed. It has four leads: red, black, yellow,
and green. I guess that yellow is the RPM sensor and green is the high/low
voltage sensor.

I tried using a 5-ohm resistor as well as a zener diode on the red lead.
This did not work because the equipment senses something is wrong, and the
fan won't spin. To try another strategy, is there something I can place on
the green or yellow lead to trick the system into reducing the fan speed?