I once had an engine failure which was caused by one of the inlet valves sticking open, which presumably screwed up the induction to the other 3 cylinders. In my case the inlet valve remained stuck open and the maintenance guys were able to confirm the cause of failure. Possibly in your case the valve freed itself which may explain why the engine operated normally after your field landing. If this is the cause, if its stuck open once it could stick open again..........