In that case current draw is likely small, still to use resistors you need to know current draw when running. Diodes will drop the voltage by approximately 1.5 volts. So instead of 4v one diode would give 4.5v and two diodes in series 3v. 3 diodes of course would give 1.5 volts. I guess it depends how close you want to be or just see the effect of slowing the the fan.
Pete