Hi Widget,

Are you sure it's 4 mm and not 3.96875 mm which is a 5/32 plug ,https://www.ebay.com.au/itm/175711111509

If it's 5/32 you may find a cheaper one than the one on the link as that was just the first one I looked at.

$8. for a plug you may as well spent a little more and get a complete carby if you can't find a cheap plug.


Cheers
Max.