I have a 2000 chevy blazer the blower works on 1 2 and 3 but it wont work on 4 I changed the dash switch and the resister but that didn't work

There is a relay in the under hood fuse box , blower motor relay .When switched to high it energizes the relay allowing power from the HVAC fuse /30 amp to flow direct to the blower motor. from pin 30 to pin 87 , if you look on the side of the relay there is a diagram .For all other speeds the current flows from the switch through the resistor through pin 87a to pin 30 on to the blower motor ! Check the HVAC fuse in the under hood fuse block .IF the fuse is good check for continuity between the fuse side with out B+ that means pulling the fuse and using a volt meter to see which side has power and checking the side that doesn't have power to pin 87 at the relay socket ! Probably a bad fuse or stuck relay . Pull that relay and try a different one. You can get wiring diagram at www.bbbind.com click on technical information ,then click on Technical Service Bulletins & Wiring Diagrams .then on next page go down to the bottom and click on I agree . next page click wiring diagrams ,then just fill in you vehicle information .then HVAC system . click on diagram, then you will see 7 links , click on the 5th one and that's the diagram for the blower motor circuit !

Replace the relay. It is black, has five pins.

THe only things left are the motor itself or maybe the wiring for the 4th position...