If 3 and 4 are sparking well, but make no difference to the engine note and are not soaked in fuel it suggests no fuel is getting through, but then it's incredibly hard to figure it running on just 2 cylinders.
I would take off the carb concerned, and check the float isn't stuck, as this will prevent fuel flow.
Ace carbs btw better than webbers imo, what chokes you running 36 38 ?
tabetha