RPJ II Comfortmaker not igniting but turns on

I have a RPJ 2 Comfortmaker furnace. The problem is that it is not igniting. It turns on but is only blowing cold air. I was told to call the gas company to light it but after further inspection it is showing 4 flashes: open high limit switch. I have no idea what this means, where it is located and can this be fixed by me? With limited funds I cannot afford to call in a repairman if I can do it myself. Thank you.