Quoting belieze1334:
It is impossible for one of the pair to fire and not the other. That is, if 4 is having a spark issue then so to is one. The only exception is if one of the plugs is so badly melted that it forms a short and carries current without an open spark. So, if 4 is missing, but not 1, then it is almost certainly NOT an ignition issue
... you should get out to the shop more often ...
As a tech, I can tell you we see it all the time.
Mostly, it's glazed plugs, but it can also occur from burned coil terminals.
It'll throw spark sitting on the valve cover, but put the plug back into the hole and put some compression pressure/increased density into the mix, and all bets are off.
A plug will fire at less that a couple of kv in free air, but it takes many thousands of k.v. to toss a spark under pressure.
There can be many paths that have less resistance than that voltage level.
The spark energy can trickle down the side of the ceramic, due to the coating of carbon that's left on a "fuel fouled plug" after the fuel is burned off and a layer of carbon glaze is formed on the ceramic.
And, my personal favorite is when the coil tower/plug wire corrodes over time, and eventually the resistance gets high enough the spark punches out the side of the terminal. (I've got a pic of that failure on my old laptop, and I've seen it several times on these cars.
Both of those scenerio's present a single cylinder misfire in service, and yet will often pass the ole spark plug on the valve cover test.