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Welcome to JA.
You might have a head gasket leak. Do you ahve any sigsn of a leak. Is your coolant level low. Look inside the oil cap to see if you see any creamy milky substance. The only misfire code you are getting is for cylinder number 4. The reason the check engine light flashes meaning there is a catalyst damaging misfire.
Yeah you could have just a leak in one spot on the head gasket. Also look at the spark plug when you pull it out to see if it looks steamed cleaned that means that possibly coolant is leaking into the cylinder. You will have an o2 sensor in front of the cat and one behind the cat to monitor making sure the catalytic converter is doing its job. Its sounds like you have covered everythingelse so I would get a compression tester.
I am not sure exactly what I mean tell me if I am wrong. If it is arcing out of the spark plug wire at the boot on the coil then the boot may be split. arcing you could try switching the wire and a coil to a different cylinder to see if the misfire moves. Do them one at a time so you know what the problem might be.
The wires are different lengths you can switch them it is not going to mess anything up. I appreciate it. Let me know if you have anymore questions and please leave positive feedback. Thanks again