My funace reads 4 what dose open high limit device mean and how do I repair it TRANE

Open high limit can be

Dirty air filters
Closed registers
Blocked cold air return
Blower motor problems
High limit control defective or out of range.

Look on the inside of the blower door for a wiring diagram. It should point the way to the high limit control. THIS IS A SAFETY DEVICE. Do not jump this control out of the circuit unless you are testing and then only if you have the proper equipment/tools to do the job.