All the stuff Ive read suggests 5 or bigger the is best for snapper. Although as Hangler quite rightly says says 4 (and a 2 or 3 keeper) are the best for gurnard they will also take a 5 quite readily as they have big mouths the old grunters.

Another idea if the snapper are on the bite and they can be really busy at the BOP is to try kahawai instead of conventional baits. Ive heard that the bigger ones hang out the back and can be more partial to that. Worth a try anyway.