What does code P304 mean?

From Google not sure. "A P0304 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #4. A code P0304 may mean that one or more of the following has happened: Faulty spark plug or wire Faulty coil (pack) Faulty oxygen sensor(s) Faulty fuel injector Burned exhaust valve Faulty catalytic converter(s) Running out of fuel Poor compression Defective computer"

Simple. Your #4 cyllinder misfires/d (all it takes iis a one millisecond error to flag the CL!). #4 is nearest the driver's seat (left rear plug). First check that the wire boot is fully pushed onto its plug (this takes a little effort). Then check the wire plug at the coil end: you'll often find a wire running too closely to the metal coil base and/or the intake manifold. Just twist the boot a few degrees to drape it away from metal. Otherwise it can "spark" intermittently by shorting to ground in wet weather. Lastly, if boots are tight and drraped nicely and PO30X appears again, remove the indicated spark plug and check for old age or a LOT of oil on it from leaky valve cover seals. If old replace all 4 with the new cheaper iridium ones. If wet pull all the plug wires and wipe them; then add a bottle of stop-leak sealer to the oil and wait a few weeks for it to expand the weepy seals and keep the plug(s) from fouling. Also NEVER use 5w oil, as it'll seep through all these seals too easily. Use 10w if you have winters; otherwise 20w50 if in the south. VERY RARELY will an actual ignition cable be intermittent because an idiot wrench pulled on one too hard and separated the inner fiber conductor from the boot's plug metal clip. So if all else fails spend $40 on new wires. Easy peasey.