Consider testing the injector before buying a new engine. If you have an ohmmeter you can test the injector by testing each electrical lead on it. Alternatively you can swap injector #4 with another injector, pull the battery negative cable for about 5 mins, then reattach the battery cable. In this way you can determine if it's an injector issue.
If not, P0304 can also arise due to:
If not, P0304 can also arise due to:
- Open or short in engine wire harness
- Connector connection
- Vacuum hose connections
- Ignition system
- Fuel injector assembly
- Fuel pressure
- Mass air flow meter assembly
- Engine coolant temperature sensor
- Compression pressure
- Valve timing
- PCV valve and hose
- PCV hose connections
- Air induction system
- ECM
(I know, that's a laundry list)