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Try this test and let me know what you come up with.
Swap positions of the #3 and #4 plug wires at the coil pack. Clear the codes, then see if the code that returns is still P0304, or changes to P0303. If the code changes to P0303, replace the coil pack. If the code remains a P0304, the coil pack is OK.
If the coil pack has been ruled out, swap the #4 injector to one of the other front cylinders, and see if the code follows the injector.
If the miss remains on #4 cylinder, do cranking and running compression tests.