1999 Chevrolet Suburban 5.7L - P0304 Cylinder 4 Misfire Diagnosis and Solutions

I recently purchased a 1999 Chevrolet Suburban with a 5.7L V8 engine. The vehicle was originally sold with check engine light off, and the codes were cleared. However, during a smog inspection, a P0304 misfire on cylinder #4 code reappeared. I've reviewed numerous forum posts on this issue, and an older owner mentioned that someone may have replaced an injector—however, I confirm there are eight injectors in the fuel rail system, so no such replacement occurred. The engine runs smoothly under all conditions: strong power, consistent performance at both high and low speeds. The only noticeable issue is a slight misfire during idle, which does not affect drivability. I've already replaced the spark plugs, but the P0304 code persists. Below is a photo of the spark plug removed from cylinder #4 for reference. Given that the engine runs well otherwise and the misfire is minor, I'm seeking advice on potential root causes—such as faulty ignition components or fuel delivery issues—especially considering the age and mileage of this vehicle.