This is caused by failure of some resistors in a resistor pack which can be got at by removing the glove box assembly. You can buy the correct resistors on eBay with instructions on how to fit them. If you can solder things and use a screwdriver, you'll be fine. Speed Setting 4 will only fail if the heater blower itself develops a fault. This is an easy DIY job!
Mike A.R. Powell, November 2008