#4 buck is pretty small, but i would use #4 buck for deer, but thats only if the following apply,
1. the deer was no more than 30 yards away walking slowly or standing still with a clear shot.
2. it patterend great from the shotgun i was shooting it with, meaning atleast 8 pellets in a 9" paper plate at 30 yards. and a few more within a 12" circle.
3. i only had #4 buck with me. i only carry a few rounds of it.
i carry 10-12 #1 ,a few 0 buck and 5 slugs. unless i was using the VR barrel. NJ laws says barrel must have open iron or peep sights or a scope on the shotgun, if in possesion of any slugs.