Check spark plugs, wires and coils, paying particular attention to cylinder #4.
If you suspect a coil, switch the coils around and see if the problem moves with the coil for cyliner #4 (do you have a scanner?).
In the dark, with the engine runnin, in park. high revs ... mist the engine compartment with a spray bottle of water and look for blue sparks (electrical arcing). This may tell you if a plug or wire is arcing to ground.
If you haven't changed plugs and wires, I would start there.
If plugs, wires and coils are all okay, switch the fuel injector from cylinder #4 with another cylinder and see if the misfire moves with the injector.
My guess is the plug wire or spark plug on cylinder #4.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)