Fan switch question on 1993 Ford Escort

I have a 1993 escort the heat works fine the air doesnt work at a but not worried about that, the fan works on 1 2 and 3 but the whole system shut down when you put the switch on 4. I have replaced the switch and the fan but didn't fix anything any advise from anyone?

Asked by for the 1993 Ford Escort
Sounds like a blower motor resistor - this is fairly common on this vehicle. Actually, make sure you verify the wiring and connector is ok as well, as they can melt. This vehicle also had a wiring harness issue much further back at the fuel pump, where a jumper harness was installed with a fuse to prevent a short that could melt wiring into the dash and beyond. Not likely to be your issue, but just get a good look at the wiring. The resistor is nearly a 100% fix for your concern.