How many parts on #4 carb are Not interchangeable with the parts on one of the other 3 carbs? Suzuki doesn't build the carbs, they buy them and it would become costly to order them all significantly different, they generally avoid that. I suspect you just need to try harder, or you might better start shopping for replacements, but sets of 4 carburetors at a time is going to get expensive real fast, new or used. New carbs are very costly and used carbs will need just as much repair service as your old ones did.
You're still lucky the originals are not CV carbs, that would have made it even more complicated, the ones you are working with are relatively simple.
best luck getting it firing on all cylinders