Our 2004 Ford Explorer XLT 4.0L V6 heater stopped blowing air on settings 1, 2 and 3. Setting 4 was blowing hot air fine and we thought it was just the switch so I went to buy a new. Another customer overheard and suggested we check the blower motor resistor. My husband hasn't had a chance to pull the old one yet and now the heater is still only blowing air on 4 but it is cool air until he's been driving for at least a 1/2 hr? Could it still be the resistor or would that not cause it to blow cool air?

 

It seems your vehicle has two separate problems. I shall try to explain,
The blower speeds 1,2 &3 are controlled by the resistor in the blower motor circuit. Number 4, is hi speed and is supplied by a separate circuit through a separate relay.
Replacing the resistor should take care of the blower speed problem.
Now, comes the good part, there are two possibilities for the air being cold. The first could be an engine thermostat that is stuck open and not allowing the coolant to reach the proper temperature before opening. The other possibility could be what we call the blend door . That door is controlled by the temperature setting with the heater temperature control. What might help to confirm the engine thermostat not working properly would be to feel both of the hoses that are in the engine compartment that go to the heater assembly inside the vehicle. If they are hot, the thermostat is probably OK. If not HOT, the thermostat is probably stuck open and needs to be replaced.
If the first part is true and both hoses are HOT, there is a good possibility the blend door is stuck. I left out one other possibility, and if the heater core, like a radiator in your home, is restricted with sludge/debris that would prevent the hot coolant from circulating enough volume to provide good heat. I hope I provided enough info for you to tell your husband what could be the problem. Now you can tell him it's his turn, now fix it!
Oh!. Merry Christmas !