are you running low octane fuel?? If the other 3 plugs were showing signs of overheating/melting etc I’d be worried that you are on the cusp of LSPI !! but as the other plug appears to be ok, its possibly just a faulty spark plug , are you running std heat range plugs? maybe one stage colder might suit?
No Mods. No rockers. Just a chunky knob, thats now been replaced by an RS knob innit.
Std as Ford intended, but with a space saver wheel and jack and nuts and wheel brace. oh.. and flaps, a man has got to have flaps.