Sooty Spark Plug

[Jall]

A proud new owner of a lovely Fulvia 3 (with very little experience working on cars):

Since becoming the new owner of this car I have driven around 400 kms. Apart from some electrical issues (opened another topic for that), I have noticed a problem with the engine throughout the last 2 drives:

I had some difficulties to start the engine and noticed that it was running a little rough while driving it. I checked the spark plugs and noticed that a single one (cylinder nr.4) appeared black and sooty.

I suppose if the mixture was generally to rich or if I was the culprit with a wrong use of the choke that all 4 spark plugs should appear sooty.

What could be possible reasons and is there anything I could check before taking the car to a mechanic?