Fig. 3 Ford 5.4L Engine Firing order: 1-3-7-2-6-5-4-8 Distributorless ignition system (one coil on each cylinder)
Get a spark tester,connect it to the spark plug wire and check for spark,if spark is present,then probably the spark plug is bad.
Also could be the spark plug wire itself.
If cylinder is not firing,plug will foul...meaning,if the spark plug wire is faulty and /or the coil is faulty,it will foul the spark plug.
If the fuel injector for that cylinder is bad,it can foul the spark plug as we..
Also ,if the cylinder compression is low,it will run rough....
Not a bad idea to go ahead and tune it up,replace the spark plugs and wires...go from there.
Just remember to Gap the spark plugs correctly to manufacturers specs.