There is a difference between killing somebody and using deadly force. I don't think anyone particularly has the legal right to kill anyone in civilian life. "Homicide" is what you did, period. After the fact, "somebody else" may decide it was "justified".
On the other hand, if you have the moral and legal right to use deadly force against somebody and he happens to die as a result or your legal use of deadly force, that comes under the Law of Tough Nuggies. If anyone does not understand this they should take a CCW class or study up on your State's laws. While you are studying your State laws, pay particular attention to the definition of revenge killing, mayhem, and give some serious thoughts about any bravado comments you may feel like making to witnesses and investigators.
Also keep in mind that that use of less lethal ammo or shooting to wound is still deadly force in the eyes of the law because the MAY kill and they put the recipient in fear of his life.
According to FBI penetration tests, some weak loads of 4 buck do not have the required penetration.
Theoretically, the ammo you chose should make no legal difference, but we don't live in a theoretical world. If you live in a pro-crime legal jurisdiction, have a bad lawyer, are the nervous type, etc. you might want to worry a little about this. For most folks in most places with a ''clean shoot, cap'n Furrilo" shootout, just get any common name brand buck or personal defense buck and don't worry about the details of the ammo.
000 and slugs will work just fine, but they have a tendency to over penetrate. Unless you live in bear country, they are probably not the best idea.
If your concept of home defense is to encourage a bad guy to go away but only have a 50% chance of stopping him if he decides to charge, use birdshot. In all other cases, use buckshot!
20 gauge buck is easily available online.