P0304 code is displayed

Were the spark plugs checked or replaced? It could also be from a fault with the injectors. Have those checked. Apart from these, it could also be from loss of compression.

Test one, remove both the one and four plug wires. Turn over the engine while watching the coil pack. It will spark between the towers if it is firing. (As it is only showing P0304 it is likely firing as that engine uses a waste spark system that fires two cylinders at the same time off each coil).

Next pull the plug wire from number four and hold it close to the top of the plug (use insulated pliers, not your fingers!) Look and listen for the spark. No or weak spark = bad wire.
Good spark = bad plug.
If the above checks out you will need to test the injector to see if it is putting out fuel, and/or check for compression.