I have OO Buck in the 12ga's at the moment but I keep some #4 buck around too.
The benifit to #4 Buck is that you have more pellets... duh right. So, lets say you had to take a shot down your hallway, which may be more than 12 feet... I'd feel better knowing I have 27 or so pellets filling the hallway rather than 9-12.
And here's another thing to consider... wether you choose OO Buck or #4 buck... the guy you shoot isn't likely to get up after either one.
One example of the usefulness of #4 Buck... small, but an example none the less. The Durham, NC SWAT team uses #4 buck as their shot of choice. I know this b/c my father is the Armorer there.
Regardless, just make sure you hit the perp down the hall.