You most likely will have a bad blower resister........
it may be the switch have it checked out
The usual cause is a failed blower motor resistor.
Its not the relay, it is the blower motor resistor.
Typically this points to a defective blower motor resistor.
Defective blower motor resistor pack.
Behind the glove box on the passenger side. It's only 4 screws to remove the glove box and then 3 to remove the blower motor
Its probably a bad resistor.
Blows on #4 only? I would replace the resistor. #4 speed by-passes the resistor.