What causes trouble code P0304 on a ford ranger 3.0?

Answer 1

Cylinder #4 misfire. The cause is usually in the secondary circuit. (You can often smell the catalytic converter trying to process the unburned fuel. If so, that confirms your problem's electrical, not in your fuel system.) In earlier models, typical cause is high-resistance (bad, internally broken) spark plug wire. In later models, typical cause is a bad coil (coil on plug--COP) for engines so equipped.