Around town riding probably isn't good enough to accurately diagnose.
Because one plug is different than three others, I would suspect the three are set similarly, and one not so much. You need to figure this out first, only then begin mixture testing and changes to get all the plugs the same. Float height, varnished or blocked fuel circuit, plug gap, weak spark....all of these could cause that one to be different than the rest....find out why.
Youll need to accurately follow plug chop steps to determine changes needed, starting with WOT and the main jet setting.