Glow plugs are like little toaster oven coils that stick into your cylinders. When commanded by the PCM, they glow red hot heating the air in the cylinders to around 1200*F in a few seconds. This is what helps achieve enough heat for the compression ignition needed to run the engine.
One bad glow plug will not prevent a truck from starting. It will just fire on 7 other cylinders, make quite a bit of noise and white smoke until number 8(or in this case #4) catches on its own. So while your in there(assuming you want to tackle the repair) have somebody turn the key for the "wait to start" light to come on and check for around 10 volts at the glow plug harness.
As far as location the glow plugs are not external. They are located under the valve covers. If you choose to do this yourself, treat this area like your in for surgery. The High Pressure oil rails are in there as well, and the plastic ...crap they use for the plug and harness assembly in there are very sensitive and prone to breaking when roughly handled. If I remember right there is a special tool that is required to pull the glow plugs... but I can't seem to remember. Your local Ford Dealer should know. And if your going to change out 1 glow plug(assuming this is still the problem), change them all. IMHO, its not worth having to go back in there again and again in a short period of time to change them one at a time.
