We have a 2009 Cobalt and the CEL is always coming on with the PO304 (misfire #4). We have had the spark plugs replaced, the coil packs, the fuel injectors, did a compression test (was fine) and cleaned the system out, replaced the intake manifold gasket (was leaking) and it still constantly comes up with this code. The car runs fine, but idles bad. Especially with the A/C on. We have tried everything! I'm tired of dumping money into this car. Has anyone else had this problem or have any insight as to what it may be and how to fix it??? I'm at my wits end and my wife is ready to kill me if I don't fix this for her.