ignition problem-coil on plug ignition type

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car started missing more and more over ashort period of time. with 52000 miles i suspected spark plugs so i replaced them. The problem remained and a scan showed p0304(misfire at cylinder #4). I took the coil from #4 and switched with #2 and the code changed but engine was still missing. Does this sound like a coil assembly and wondering possible causes. Please advise.