One method: Don't used the 3 pin plug (especially if you have modified and connected the 4 pin plug). Move the pins around in the 4 plug socket (yellow 12v -&- red 5v=7v, red 5v -&- black Gnd=5v). You only have to move one lead to another position for a 5v or 7v conversion. Forget about RPM monitoring after you drop the voltage, it just won't work.
A pin extraction tool helps (thin tube), otherwise push in the pin flanges holding the pins in place and back the pins out of the socket. Try 7 volts for a decent amount of air. I bend the wires back and run them up inside and through the female side of the pin (bending the pin open & then closing them again) so they fit neatly into any of the 4 holes (two of the holes don't need the wire folded back, the molex connector has a flap which just snaps open).