Howdy,
 
Just playing with the gunson colourtune, all mixtures turned out set correct, however I did spot that cylinder #4 is misfiring at idle, and has a very oily spark plug. I thought the ticking sound up there was a tappet misadjusted, however it is certainly a misfire. Fires about 1 every 6 cycles at idle but sounds OK once RPMs are up.
I have changed leads and misfire stays.
Is the next step is check compression?
 
Cheers,
 
Rob

Absolutely

If comp test comes up ok, check your valve stem seals. I recently had a similar issue and turned out to be valve steam oil seals.
 
Rhys