Diagnosis
The F4 error code is a door switch failure. One or more of the three door safety switches is not toggling correctly when the door closes.
How to Fix It
First, verify the door is fully closed and latched. Clean the door latch area with a damp cloth to remove any food or grease. If F4 persists, the door switch assembly must be replaced. This is a safety-critical repair. Contact LG for a service technician.
If That Didn't Work
The F4 code can also be triggered by a failed main control board not reading the switch signals correctly. A technician would need to diagnose the board.
Heads Up
Do not attempt to bypass the door switches. The microwave will not operate with an active F4 code as a safety precaution.