usually codes p030# mean cylinder misfire. last number, yours being 4(p0304) means cylinder 4 is misfiring. check coil pack if it has coil pack, and make sure, (since you jus put in the engine), you change plugs and wires AND did not connect the wires out of firing order. email or call for further assistance. charlyboy_979@yahoo.com 336-712-5285 sam...
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.