Renault Scenic car miss fire

my car has a miss fire pluged it into computer came up miss fire on 4 and miss fire low fuel and gear box fault im sure the gear box fault is an old fault as nothing wrong with gear box the car was smelling of petrol before this happened and the plugs were wet. This is what ive done so far:fully serviced car changed coils round then pluged back into computer still states miss fire on 4 so i took injectors to be checked come back nothing wrong please any ideas ! could it be ecu if so whats the cheapest way to sort out thanks