Whenever someone mentions defensive shotgun loads, it degenerates into bickering about overpenetration.
When you shoot someone with a shotgun, you're hopefully hitting them with several little lead balls intended to kill them. You can argue whether you want 000 buckshot (8 x 0.36" balls in a 2.75" 12 gauge), #4 buckshot (about 30 x 0.24" balls in a 2.75" 12 gauge), or something in between. People aren't going to settle on a shell, just like people probably aren't going to agree on whether it's better to shoot someone 8 times with a .45, 15 times with a 9mm, or 30 times with a .22 mag. The bottom line is that they're all going to get the job done. If you're using 000 buckshot you have a greater chance of penetrating a heavy leather jacket. If you're using #4 buckshot you have more hits, so you have a greater chance of hitting vitals.
Regardless, overpenetration is always going to be a concern. Even your #4 buckshot, if you use Syntax360's figure of half the shot penetrating 12", will easily have all the shot penetrate a human enough to hit something important, and even if that person is wearing a heavy leather jacket, there is the possibility of it penetrating the jacket, penetrating soft tissue, and penetrating a little kid in the background on the other side of the street.
Looking at theboxotruth.com, you can see that even a .22 is going to penetrate four layers of drywall. The bottom line that it is easier to penetrate drywall than it is to penetrate people, so there is no such thing as a good defensive round that is safe to fire with your children behind the target in the next room. Anybody suggesting birdshot as a defensive round is a moron (I saw it on a forum yesterday).
I guess #4 could be a little light for someone with heavy clothing, but I think #2 or #3 buckshot is a pretty good sweet spot. You've got a lot of shot, and it still has excellent penetration vs. people. If #2 buckshot doesn't penetrate someone's clothing, well, 00 or 000 is going to penetrate better, but you'd probably be wanting a higher velocity non-ball round for that kind of situation. Still, if I only had a shotgun and I was planning on trying to shoot through car doors on a regular basis, I'd go with the 000.
For home defense, I'd easily trust my life to anything from 000 to #4 buckshot, though. I wouldn't bother arguing about it or overthinking it. They're all great. My argument would be that since they're all more than adequate, I'd be more concerned with having a gun that's easy to handle. I'd suggest an 18" 20 gauge pump gun -- a lightweight gun that still makes the crap-your-pants slide-racking sound.