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Hello, I hope I can offer you some assistance.
This fault is usually due to the heater fan resistor failing. When the fan speed is set to number 4 then the resistor assembly is by-passed which is why the fan works on this speed. The resistor is up behind the dash, if you look up from the foot well behind the glove box you should see it on the heater box, it will have a connector with 4 wires to it and should be held in place by a screw.
Let me know if I can assist any further and I'll reply as soon as I can.