2003 Ford F-150 engine

There's no 7.4L for your truck. Based on the spark plug, I'm going to have to guess you don't have the diesel engine. A common cause of misfires on Ford V-8 truck engines is plugged EGR tubes. Actually, the only passage that's still clear is the one for the misfiring cylinder. The amount of exhaust gas flow is measured and controlled, and when some of the tubes leading to some cylinders become plugged, those cylinders run fine because all they're getting is fuel and fresh air. All of the exhaust flow goes to just one cylinder, and since there's no room left for fresh air, it misfires.