Supposedly, #1 buckshot is ballistically the best size of buck.
It penetrates better then #4 and gives more destructive damage then 00.
However, #4 buckshot is good enough.
Famed Vietnam founding Navy SEAL Chief James "Patches" Watson used Navy standard #4 extensively in his two tours.
When asked if #4 buck was too light to be effective, Watson replied; "No one I ever shot complained".
#4 will be very effective inside 10 yards unless the target is wearing thick, heavy winter clothing and a leather jacket.