And it could be that the spark plug tube on 4 lost its seal (there are two O-rings) and it blew oil onto the spark plug - shorting it out and then vaporized the oil in smoke.
You didn't say if the smoke came from the exhaust pipe or the side of the engine. Some research I have read indicates many AOS failures have symptoms of excessive smoke coming through the exhausts (but the AOS could also be leaking on top of the engine (doesn't appear to be as common though).
If the spark plug tubes are leaking you will see oil more from the side (or both sides) and smoke coming from where that drips on the catalytic converters and exhaust system.
I would check out tube and plug on the 4th cylinder first.
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