Before you buy a resistor pull the control panel out and look for burned or melted areas on the fan speed switch. If it is melted then the only fix is to relay the fan speed switch or else it'll keep melting them away. Also check for melted wires at the resistor plug. I lost speed three and my switch had been fried. Speed four bypasses the resistor and so you most likely have a burned switch.
2000 TJ 4.0, NV3550, NP231, D30/35, 3.07's