I'm looking for the proper way to diagnose a P0304 code. I have done some searches on the issue, but I don't see any posts from people with the exact symptoms I'm having.

Cylinder 4 is the ONLY cylinder throwing a code and MIL. Also, when the car sits overnight (or for several hours), it starts up just fine and runs smoothly. However, if I turn the car off after driving it for a bit, and then start it up again right away, the engine runs very rough and the P0304 Cylinder 4 misfile MIL code comes up. When the car cools down again, the engine runs smooth...

I have so far swapped the cylinder 4 ignition coil with on from cylinder 5 (still same code). Then I tried swapping the spark plugs (still same code).

Could someone point me in the right direction? Could it be an injector problem? If so, how would I check it or remedy it? Why does a blown head gasket cause misfiring? Could that be my problem?

Thanks in advance for the help!

 

You are on the right track as far as diagnoses. Next thing would be to swap injectors.
It is possible you do have bad head gasket. Remove coil & plug from that cylinder let it sit overnight place a clean paper towel inside the area where the coil sits , crank the engine & shut it off , check the towel for antifreeze. If there is antifreeze , time to pull the heads😢