Is the car still using coolant?
If you can drive it 20 minutes without it overheating then the head and gasket are most likely intact luckily.
As far as testing the spark from the coil take the #4 spark plug wire out of its bore and using some insulated pliers, hold the end of it closely (but not touching) to a metal part of the engine (part of the block, a bolt head, etc. as long as its a good ground) and observe the spark as it arcs to the ground (if any spark is even there). It should be a bright blue color and make a loud snapping sound. If its an orange or yellow color, or no spark at all, and the plugs and wires are new, you likely have a bad coil. Zetec coils are known to crack. Perhaps some leaky coolant from that housing got on it and signed its death warrant.