Every deer I've ever shot at, I shot at with buckshot. Every one of them fell right in their tracks. They might have tried to get back up, but not one ever did. Only one took more than two shots.
I've seen a lot of deer killed with buckshot. Within it's range limitations it is as deadly as anything else. I have heard stories of deer being killed at 100 yards with buckshot, but I suspect those who tell them wouldn't know 100 yards from 100 miles. In my experience, about 40-45 steps is as far as I've ever shot at a deer. That's a long way around here. Even when hunting over a field stand, I'd step off that 40 steps and tell myself, inside here, shoot. Outside, don't.
Having said that, I would never use No 4 buck. I've heard of people who have killed deer with it but I just never did trust it. Every deer I ever killed was with No 1, all but one of them out of a 3" mag gun. I always felt this was the best combination of number of shot, and size (weight) of the pellets. A lot of guys I hunted with liked 00, or even 000. The deer I have seen killed with 000 went down like lightening hit them. That stuff breaks BIG bones and keeps on going. It just never patterned well in my gun. For me it was No 1 or nothing.