That is the incorrect sticker for a '67 model, if in fact that's what block you have??
That sticker belongs to a 73 model, which match your outer cowls. Grrr.
Where are you located? I think you have a Frankenmerc? I hate them.
As for number 4, I suspect spark, not fuel.
The bottom Carbie fuels both 3 & 4. Even a crook/bent reed would prevent 4 from receiving fuel.
Leave plug in 4 and place a grounded (spare) plug on number 4 lead. Start motor and check spark for strength.
Never start motor without pug in hole. Its starves piston, crank etc of oil, plus it will start a small fire at the back of the motor. Not a good look!