4 flashes on troubleshooting display says open high limit device, where is this device located on furnace?

Furnaces frequently have more than one sensor that trips when overheating happens. Mine was caused by repeated back drafts. Exact which model do you have? The switches are not in your face and I find them to appear almost interionally hidden so you have to call out a repair guy when most can be reset by pushing a button. The reason they want a repair guy to come out is to find out what is causing your issue. This is like resetiing a breaker that keeps tripping with never looking for a short or fault.