99 Nissan Altima blower fan works only on highest setting (#4)

Blower motor resistor probably needs to be replaced.

Sounds like the blower resistor module needs to be replaced. Could also be the switch.

The blower resistor has gone bad and needs to be replaced, it's common for it to burn out due to time. You can pick one up at Auto Zone and they will have the step by step instructions to replacing the resistor. 
The blower motor resistor is under the dash, next to the blower motor. You will need to remove the under dash panel to access it. It is very easy to replace, just two screws hold it in. May take about 15 to 20 min with simple hand tools. 

Good luck and hope this helps. 

You have a problem with your reostat. This is located under your dash behind the glove box and beside the blower motor. Unplug the plug and take out two screws hold it in place. Take it to your local recycle yard and get a good used one make sure to ask if it was tested and if it has warranty. Please let me know how you make out.
Thank you

I've had mice in my Nissan PU and actually caught one in a trap set on the floorboard; so I know they can get inside. They also knocked some of the fiberglass insulation into the blower motor so it vibrated as it was no longer balanced. You could have a dead one in your blower. Mine was easily accessible from under the passenger side under the dash. Three screws and the motor dropped so I could clean out the fiberglass.

Replace your blower motor resistor....

It is very possible you blew a fuse. There are two 15 amp fuses for the blower motor, make sure you check both of them.

if the blower works on high then its not a fuse it could be the switch or the resistor that cuts back power for the lower settings

Hi.....
On your Vehicle the problem for the FAN speed is NOT the FAN switch.
99.9 % of this problem on cars is the BLOWER MOTOR RESISTOR.
The blower motor resistor give the speed to the Fan.
But on High speed the switch make a By pass of the switch using a Relay for the high speed.
The Blower motor resistor, is located Beside of the Blower motor On the passanger compartment.

If you replace the blower motor resistor you have 99.9% of this will solve your problem.

I'm a Mechanic.

I hope this help on your problem.
Thank you for use fixya...