So I bought the car and I totally forgot to check the A/C it works fine, also the heater, but it only work on 3&4 not 1&2. Why is that? How can I fix it?

 

If it's a 1997 or older, the problem is probably the blower motor resistor. It can be reached by removing the glove compartment. You should also remove the blower motor and check for a mouse nest in the ductwork. Lack of airflow will burn out the resistor.