More than likely you're not getting fuel to #4. But, double check the plug caps to the coil and spark plug are fully seated. Then, open the drain screw underneath the offending carb and catch the fuel that flows out. Any debris?
Start the bike, and use the handle of a screwdriver and rap on the carb body. This can help dislodge a stuck float or stuck "closed" float valve. It's not uncommon to agitate debris that's accumulated in the gas tank after years of sitting idle on maiden voyages. Is there an inline filter of have you fully cleaned and flushed the tank to verify no corrosion?
Pipe should be equally hot within 30-60 seconds of firing. Use a spray bottle of water directly onto the header to avoid scalding your hand, but test them for temps promptly. If they're hot, and you ride but the bike quits, pull over and test the pipes each. If a pipe continues to work, then not, probably a float issue in that carb (wrong height or debris as mentioned).
Eliminate these possibilities before moving on to igntion, and set your idle once, after hot and leave it be. And vacuum synch your carbs soon.