The engine diagnostic code P0304 means that your engines number 4 cylinder is not firing properly and has a misfire which could be caused by bad ignition coil, bad spark plug, bad spark plug wires, or even a problem with the fuel system.
The cylinder could be losing compression as well but it's hard to say exactly what is causing it without putting the vehicle through a better diagnostic test.
A regular auto shop should have the equipment to run a complete diagnostic test although it might cost you some money for the test but is well worth it to pinpoint exactly what the cause of your cylinder 8 misfire is.