4" is more than enough, you can get away with a snubbie no problem for home defense. the lighter, and the shorter the barrel the better for HD and carry imo. the longer the barrel the longer the sight radius you have which can lead to improved accuracy, but if you practice enough you can be very proficient with a snubbie. a longer barrel also improves burn time for the powder which increases velocity, and in turn energy, but for HD you really dont need a lot of either to get the job done. besides you can get rounds that are plenty hot enough that they still produce a ton of velocity and energy out of a shorter barrel (not as much as you would get out of a longer barrel with the same round, but still plenty to do the job).