My air conditioner/heater blower will not work on the 4th setting. Setting 1,2, and 3 do work. I have replaced fan speed switch and replaced the resistor. Any ideas?

You'll have to find the correct wiring diagram in order to trace down the fault in your system.. Once you obtain the wiring diagram you'll be able to see what else beside wire is between the #4 speed setting and the blower motor. My guess is you might find a high speed fan relay is triggered from the #4 fan switch setting, applying full battery voltage to the blower motor. If locating the appropriate diagram proves to be difficult you can try looking around the underhood and passenger compartment fuse panel to find a relay. Keep in mind not all relays are always in fuse boxes which makes the diagram important.