My recommendation would be to take a comprehensive defensive shotgun course and listen carefully to what they suggest is most appropriate for home defense. Gunsite (Arizona) offers a one week course that's excellent. I haven't looked but suspect Thunder Ranch (Texas) offers a similar program. Effective handling of the gun is more important that the specific load used IMHO especially if the confrontation is indoors.
A good common-sense load would be #6 shot, 2 3/4 shell, high brass. Just a good bird round available everywhere and a safe bet. Plenty of power at indoor ranges.
As good a choice as any (#4 or #6). I haven't seen anyone post another good reason for using "duck loads"..., that is, when you are sued by the perps' survivors, you won't have to face their attorney "grandstanding" for the jury (e.g. "And tell me Mr. Smith, just why did you have X loads (take your pick) in your shotgun? Obviously you were seeking to kill someone weren't you?)! You will be far more sympathetic when you can say you just had some bird hunting shells for your shotgun!