4x4. What would make heater blower work sometimes?

A little more info needed..blower works but blows cold air, blower doesn't work?

check coolant level --blend door --control valve to heater core --possible heater core clog --flow to heater core problem

Bad blower motor controller

A Chiltons/Haynes repair manual for you make/model/year will lead you through the process. Have you tried replacing the fuse?

What have you checked so far ?
Fuse - resistor - power at the blower motor ?

if radiator hoses are the same temp at opperating temp and heater hoses are the same the blend door in the h vac may be the problem

If the blower works, but no heat comes out, and the heater hoses are both hot, it could be a problem with the blend door under the dash or the HVAC control module.

replace the blower switch you control by hand on the dash

voltage regulator replace it

Do you mean the fan does not work or the fan blows but there is no heat ? If you mean the blower, check the resistor at the wiring just before you get to the blower.The resistor is a small gadget. Not to be confused with an electronic resistor. If the blower works but there is no heat, replace the thermostat.