Did a service today and removed the spark plugs to clean and check gaps. I noticed that cylinder 4 plug looked quite dirty and the outside electrode was partially burnt away (see pic) although all other plugs looked fine (clean and fawn coloured). Then I recalled I had the same issue in 2017 when I did the plugs, although the outside electrode, while worn, wasn’t quite as burnt away.
Has anyone got any ideas what could cause this, injector, coil pack, other.