Transmission or transfer case
Some automatic transmissions will fail safe to 4th gear. If this is happening then your transmission has a problem and needs to be repaired. The transfer case has nothing to do with the transmission shifting. All the transfer case does is sends power to the front and rear driveshaft from the transmission. Are any check engine lights lit up?
sometimes a bad TCC, or torque converter control solenoid will cause this. of course scan as bob suggested, but also, if you can google the location of the tcc, unplug its wire connector and see if the problem "fixes" if so, it is your tcc. You can drive the vehicle with the tcc unplugged, but you will sacrifice mileage and the cruise may act strange.
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