Winchester #1 buck. Patterns well and works on deer.
What are you basing you assertion that 4 buck is marginal? Ive seen alot of deer killed with #4 buckshot. Most were on the move in front of hounds. If the pellets hit their mark the deer went down quickly. I killed a few with #3's out of a 20 when I was a kid. Killed a bunch more with #1's cause that what my shotgun patterned best. Unless a bad guy is wearing body armor, there is no way that a load of #4's center mass at 50 feet or less wont be devastating. If it will kill a deer at 30-40 yards, it will certainly kill a man at half that distance.