The engine control module has detected a misfire specifically in cylinder 4. A single-cylinder misfire is typically easier to diagnose than a random misfire because the problem is isolated to the components serving that one cylinder.
Find certified mechanics near you who can diagnose and repair P0304.
For informational purposes only. Diagnostic trouble codes are starting points for diagnosis, not definitive answers. Always consult a certified mechanic for proper diagnosis and repair. Estimated costs vary by vehicle, location, and shop rates.