From the Heater AC.pdf file repair manual (found here
, check for post by Denver1 and follow instructions to his mydrive site)
BLOWER MOTOR DOES NOT OPERATE
Check for blown HTR fuse, heater main relay or blower resistor. Check for faulty A/C controls, blower switch
or blower motor. Check for faulty wiring harness, grounds or connectors.
Could be that your relay is giving faulty readings or the switch at your dashboard is very worn/ broken (doesn't change the speed at number 1,2 and 3 but does work on 4).