2001 gmc sonoma got heating problems. When the heating control is set on 1 or 2 it has warm heat when set on 3 or 4 loose heat

When you say it loses heat, do you mean it starts to blow cold air? Or it stops blowing any air?

It kind of sounds like it could be one of your relays, I believe they are located behind the glove box, or its always possible that your switch itself is going out, but the relay would be a much cheaper place to start.