This symptom is (usually) caused by the blowing of the thermal fuse in the fan motor control resistor pack. (The resistive power drop is a really cheap way for Ford to control the current, and hence the speed of the fan).The reason the fan works on setting 4 and not on the other 3 settings is that the full speed setting 4 bypasses the resistors and the thermal fuse.[Those with A/C would also find that the A/C doesn't work at all as it only works with fan speed settings 1-3.]
This resistor pack is quite awkward to get at as it is located behind the glove box and some ducting for the heated air. The reason it is there is that Ford use the airflow from the fan to cool the resistor pack (which can get pretty damned hot!). However, with some squirming around on your back, a long shaft Philips screwdriver and so on it can be removed and replaced by a "normal person". [The Focus Haynes manual has a small section detailing the heater blower removal and the location of the resistor pack]
In my case I had the A/C and the fan stop working on settings 1-3 a few years ago. The local garage replaced the resistor pack and everything was good again. A month or so later the new pack failed. Initially it was suspected that it was just a faulty pack so they fitted another free of charge. Again a few weeks later this second replacement failed. (This next bit may seem a bit dodgy!) The mechanic said that he had had this happen before on another customers car and that, in some frustration, they had soldered a wire across the thermal fuse to prevent it happening again and this had fixed their problem. With some concerns I gave him the go-ahead to do this and all seemed well until the winter, when the fan blower failed again. As the thermal fuse had been bypassed it didn't seem that this could still be the same issue but ... yes it was. The resistor pack had got so hot that the solder melted and the bypass wire fell off!!
Clearly we had some other issue - and this is where the cheapness of Ford's design solution comes into play. In my case what was actually happening was that the fan was running slightly slower than expected particularly on setting 1. Some condensation from the A/C, or just incoming moisture, had slightly corroded the bearing for the fan motor so it wasn't turning as smoothly as it should. (In some cases I have heard of them squealing). As the airflow from the fan is used to cool the resistor pack eventually the pack gets too hot and blows the thermal fuse. This is most likely to happen on setting 1 since this has the least airflow and the highest power drop through the resistor pack.
I removed the blower unit and cleaned and oiled the bearing, replaced the resistor pack (again) and it's still going well 6 years later.