P0304 is a misfire in cylinder number 4. What was done to fix the misfire? An engine misfire can be caused by engine mechanical issues, fuel issues, or ignition issues. You've got to start with the basics, check the engine is mechanically sound (good even compression, no vacuum leaks,valves are properly adjusted (in the case of shim or screw adjusted valve clearances), check the engine's ignition system is good (good spark plugs coils or plug wires where applicable) then the fuel system needs to be tested and inspected to make sure it is operating efficiently. A scan tool is needed to retrieve codes stored in the computer of your car to see what is causing the Check Engine Light to come on.