2006 ford ranger 3.0 keeping getting misfire code on #4 cylinder,

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the repair shop has replaced the injectors, the main computer and did a repair on the cylinder head telling me the valve was bad. the repairs lasted about 3 weeks and it's acting the same as before. we have noticed that when we add Marvel Mystery oil to the gas it goes away, we have to do that every other tank full... got any ideas on the fix? I have 5 rangers and at some point they all act this way..