If the resistor goes bad, you loose 1-3 and only 4 works.
The resistor lowers the voltage fed to the fan for the lower speeds. On high, it sends a full 12V. It's not the motor or the resistor. It is probably your switch or the wiring.
lived here 25 years and bought my car last jan. i've never really used 4. my car is usually up to temp in about 10 mins or so. and i end up turning it down cause the car gets too hot.