I have a 2008 F-250. Only speed #4 worked on Heater fan. I replaced Resistor, 1-4 worked for a short time, now just #4 works, what might the problem be?
Maybe another burnt up resistor. This would be caused by a dragging blower motor.
Even a dragging blower that appears to sound fine and produces plenty of moving air may be drawing too much current (amount of electricity) due to wear in the blower motor.
Checking blower motor current could be done with an ammeter if one wanted to verify it. The blower will draw the maximum current on the highest setting of course.
Been There, Done That - Your Exact Scenario -
" Only speed #4 worked on Heater fan. I replaced Resistor, 1-4 worked for a short time, now just #4 works . . . "
In my case, replacing the blower motor/blower fan fixed it.
I second OK4450’s analysis.