When the blower will only work on position 4 it is nearly always the blower resistor pack.... thermal fuse blown. When the pollen filter gets dirty, less air flow, so the thermal fuse can blow at the higher temperature. Remove glove box and remove resistor pack from pollen filter housing. The resistor pack will have a copper strip/flat spring which is likely to have sprung apart as the solder joint melted and the copper strip sprung away. You can fix this by buying a 152 degree C thermal fuse from Maplins for about 80p and soldering in... bridging the gap where the copper strip used to be before it sprung. Don't overheat the thermal fuse when soldering it in or it will blow during installation!
Refit, replace new pollen filter, reinstall glove box ... and all should work again!
Clive Nicholas, February 2013