Plugs fouled with fuel is a good place to start for me. I would be under the opinion that the tips of the float needles are still dirty as well as the seats. I know it means pulling the carbs again, but there are a few tests you can do off the engine to be sure the carbs are working.
I clean the float needles by running the rubber tip ever so lightly over fine emery cloth. I also take the end of a piece of emery cloth, roll it into a cone and clean the seats.
To check whether they are sealing, I fill a lawn mower gas tank with gas and hang it 2 feet above the carbs connecting it via the fuel inlet. (FYI, I always use a valve in the gas line tubing from that tank so that I can shutoff the gas as needed.)
Once the tank is 2 feet above the carbs, I open the valve and fill the bowls. While filling, I check for leaks inside and outside.
If no leaks, next I check to see if the float needles will still stay seated by simulating riding. I turn the carbs left and right and lift them 45 degrees in both directions while shaking them. If gas leaks anywhere, the needle and seat in that carb is still dirty and will require further cleaning. Don't be afraid to give them a good shake while testing. They get tossed around a lot on the road and bounced when hitting potholes.
Once satisfied they are not leaking and the needles are sealing well, check the accelerator pump circuit. Looking down into the chambers, run the throttle out all the way and be sure each is getting gas squirted into them.
Lastly, double check the bench sync. Mike Nixon and Roady both use this method and I have found it works every time:
Look into and around the outlet port of #3 carb and locate the bypass outlet. It will be right along the line of the resting butterfly. Note the relationship of the outlet to the edge of the butterfly. Now, adjust the 1/3 sync screw so that #1's butterfly position matches #3. Flip 'round to the other side and adjust #2 and #4 so they match each other. Then adjust the left bank's sync screw so the 2&4 set match up to #3. The 2/4 sync screw is between those two carbs. The left side's screw is aft of #4 carb.