Generally go with whatever your gun patterns best with. #4's hit harder at longer ranges, and generally will do more damage than smaller shot sizes, but there are fewer of them in a shell. #6's will make a lot of hits, with numbers making up for shot size in the damage department. #5's are a compromise. Looking back, I've taked more birds with #4's than other shot sizes. But that's just my preference. Now days, I end up having a #4 as my backup round, with the shell in the chamber usually a #6.