If setting 4 is out on the turn control well.....it's not the resistor. Setting 4 does NOT go through the blower reostate "resistoer". It pulls straight 12v power...thats the problem..jeep had a brain fart there.
And yep, VERY common issue. Saidly the fix is some relays and a new control box if the melting is too bad. Yep I said melting.
Pull the heater/ac control panel out and look at the back of it where the wire connector is connected on the far right side. It will have 3 wires. One which is a little larger and green. Check for melting. Look close. Also at the other connectors for any melting.
"""""to remove heater control box for inspection""""
Remove the defrost vent cover , pull up on the far side not the middle. pops right off.
Remove the ashtray. Open it and push down on the tab on the inside and slide out.
Remove the screw behind the ashtray.
At the top of the center bezel where you removed the defrost vent cover,,,remove the 2 screws that hold on the bezel. Pull the whole center bezel off..pull right off.
The heater/ac controls will have a few screws holding it on...take them off.
Poof your done. and can inspect the controls.
READ through this WHOLE thread. And the links in post #10.
If you have any more questions let us know.