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Thinking about it again, the cause could be a failed ignition coil. These have pencil coils, one per cylinder, but they're wired in series pairs for cylinders 1 & 4, and cylinders 2 & 3. They're common failures, and if for example the coil on number 1 cylinder failed it would also have an effect on number 4 cylinder. This could result in the ECM setting a 'Random Misfire' fault code P0300, because a misfire on two cylinders, particularly if it's intermittent, may be sufficient for the ECM to be unable to track down. As I said, the ECM only knows the engine is misfiring because it receives an irregular pattern of information from the O2 sensor, and whilst this is enough for it to determine that the engine is misfiring, it may not be accurate enough for it to identify on which cylinder the problem lies.
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