There is a resistor assembly mounted on the air duct close to the blower motor. That part has failed. It is used to control fan speeds 1, 2, and 3. Speed 4 is the maximum blower motor speed, so no resistors are involved. Replacing the resistor assembly is something the home mechanic can do. The only tool needed is a Phillips screwdriver. The replacement part costs about US$25.
The blower motor is located just above the passenger's feet. Look at the wires leading away from the blower motor. Just inches away from the blower motor there is an electrical connector located on the air duct. That's the resistor assembly. It is easy to remove and replace.
Replacement is the usual remedy. However, if a spot of localized damage is seen it may be repaired with a soldering gun.
There is a good photo of the blower motor and resistor assembly in the Haynes repair manual, page 3-8. If you don't already have a copy of this book, please get one. It will be money well spent.
a) Move the front pass seat all the way back (for convenience)
b) Remove a harness connector (with 4 wires I think) running into the bottom of the blower motor.
c) Remove the 2 philips screws that hold the blower motor resistor in place.
d) Put new one in, reverse install.
Blower Motor Resistor Part Number: 27150-1E405
You can get it here:
Should be $28 at your local dealer. It's $28 delivered from discount Nissan parts sites.
Edit: If you are handy with a soldering iron, you can try to fix it yourself. See this thread