I checked on Google for the F4 error - it said that F4 talks of a failed Humidity Sensor - quite why a microwave should need a humidity sensor is a mystery to me. However - the article said the sensor should be replaced and shows a price a little below $100 - just for the part.
My microwave is very basic and it cost about $35 many years ago - it does not display fault codes - it just warms stuff.
Good Luck...
January 2016
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