Hi there welcome to just answer hope i can be of some assistance to your problem
Ok then this was a common problem on the old focus and is caused by the heater resistor assy which is located in the heated box unit.
Now the reason you loose the first three speeds is because the first three speeds go through the a resistor unit to reduce the voltage to the motor to make it go slower but on speed four it is a direct feed to the motor.
The resistor unit is an easy job to change and is locate behind the glove box and is held into the heater box by one screw as in the picture below. so if you wanted to change it your self then it can be easily done.
Some times it is possible that the wires into the heater resistor may have melted, now if they have it is not the end of the world as a repair kit is available to buy to fix it.
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Hope this is ok for you if you require any further information then please ask
I am glad to hear my information has help you please ask if you ever require any further help