Listen to the injector on cylinder 4 with a mechanics stethoscope (try a wooden dowel if you don't have a stethoscope) and compare the sound you hear to another injector. You should hear a steady ticking noise as the injector fires. If you don't hear it you need to check if the injector is getting a signal. The easiest way to do that is to take an automotive bulb that has wires on the end that can be straightened out and stuck in a connector. Pull the electrical connector off the injector on cylinder 4 and insert the bulb wires into the body side connector. Have someone crank the engine and see if the bulb is lighting in cadence with the RPM'*.
If you are getting signal, but don't hear the injector firing, replace the injector. If you are not getting signal you have a bigger problem.
You can also try switching injectors with another cylinder and see if the misfire moves to the other cylinder.
Misfires on a single cylinder can be a bad plug, a bad plug wire, a defective injector, or a mechanical issue. If you don't fix it with the plug, wire or injector, you can narrow down the mechanical issue by doing a compression test on the misfiring cylinder, and/or pull the valve cover and inspect for any damaged parts in the valve train.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)