A/C only works on setting 4

it could also be a bad fan speed switch

I agree with GetrDone, it is the resistor that have gone on positions 1 & 2. The resistor pack (which all of the resistors reside for the blower) sits next to the blower motor. Here is a link sample of the resistor pack http://www.partsgeek.com/catalog/1993/nissan/altima/climate_control/blower_motor_resistor.html. Extremely rare for the switch goes bad.