Everyone I know who uses a shotgun for home defense uses #8 birdshot so that it doesn’t overpenetrate (Yeah, I know. HUUUUUGE sample size. 
)
Is there a reason he started with #4 shot and worked his way up?
Well, that might be the answer. If #4 is a little light, then #8 would be worse.
#4 and BB size shot is excellent for hares and foxes, though. 
How often do you check your chest for dots? No, when the bad guy sees the dot on his comical sidekick (followed by a hilarious look on sidekicks face), then he knows the gig is up.