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So yes, the unit can be repaired, but it's hard to know what is causing your issue or what part you need. There are actually 3 possibilities---the magnetron, the inverter board, or the most likely culprit would be one of the 3 door switches has failed. If you have a model number any and all parts can be ordered at Searspartsdirect.com. You can't really test the magnetron or the inverter board, but if you have a multimeter you can test the 3 door switches and see if one is bad. The most common issue is that one of the door switches has failed which cuts power to the magnetron. The fact that the magnetron isn't drawing power is why the microwave shuts off after 4 seconds which can only be caused by those 3 options I mentioned before. At 3 years old, all parts are still available for this microwave.
The door switches are not, but both the magnetron and the inverter are very expensive at about $200 each usually. Keep in mind, only one part is bad---it's just difficult to track down which part that is.
Not a problem and I wish you only the best. The only good news is that the door switch is usually the problem and those are cheap.