Littered on gravel town and logging roads I've found more 3" .410 empties marked #4 than just about anything else. And you can find the factory loads in just about any small town mom and pop convenience.

I didn't get it. An old friend with a 37A .410 explained it. Single shot "full" .410s make effective low-recoil partridge ground-swatters and, evidently, #4s provide the greatest reach. Never tried #4s but it makes sense. 3" #6s show adequate core density a good 5-10 yds farther out than #7.5s through a full M-42.