2004 Ford Expedition fan switch only runs on #3 or #4, heat

check or replace the resistor !!

Blower Motor Resistor
The blower motor resistor:
Is located on the plenum chamber near the blower motor.
Has three resistor elements mounted on the resistor board to provide four blower motor speeds.
Has an overheating device (thermal limiter) that will open the resistor coil when the temperature reaches approximately 184°C (363°F), interrupting the blower motor operation in all speeds except high.
Is serviced as an assembly. The thermal limiter cannot be reset and is not serviceable.
Depending on the blower motor switch position, series resistance is added or bypassed in the blower motor circuit to decrease or increase blower motor speed.