NAC but VAG content: DTC code retreived

[Louis-Alain Richard]

Patient is 1998 VW Cabrio 50k miles 2.0 litres AAB engine.
OEM or original parts everywhere.
The code extracted by a friendly mechanic is:
16688 P0304 Cyl.4 Misfire Detected
I also noticed a low G12 coolant level in the reservoir.
Engine is smooth, but start-up is followed by a rough idle and black smoke.
When warm everything is fine. No symptoms.

What is the most likely culprit?
Blown (very lightly) head gasket?
Wire set (original but still looks good)?
Spark plugs and cap and rotor (less than 8 months old, VAG OEM parts)?
Other?

And for the sake of knowledge, how can a system like this detect a misfire in a specific cylinder? What sensor can do this?

Thanks in advance for your input,

Louis-Alain
Montreal PQ
83 urQ waiting its new ball joints from UK, that’s why I drive the Cabrio.