#4 buckshot used to be the go-to working load for social stuff. FBI and many other agencies liked it for more pellets, and reduced past-target hazard over the heavier buckshot rounds.I had it back when I was a shotgun for home defense type guy. But then the science era of terminal ballistics took over, and it became obvious that once ranges open past conversational, where you are more likely to be using a long-arm, the lighter pellet is awfully shallowly penetrating. In addition, the reality of the risk of liability (being responsible for every shot) became clear, that a wide spread is no good and bystanders are real likely to take a #4 pellet in a gunfight. Hence the reduced-recoil 8-pellet 00 that rules the roost now: better control so all the rounds are where you expect them to be, adequate penetration to reasonable ranges.
#4 birdshot is quite small. As stated by many: in an emergency you dance with the one you brung, but I'd be reluctant to consider anything over 10 yds to be covering fire and by 25 it's practically a less-lethal round on humans. At 0 - 7 yds or so (depending on your gun) its a single column so will put a moderately deep 18 mm wide hole in someone, so slow them down pretty well. At near contact ranges, even people with soft armor won't enjoy it at all, though it won't penetrate.
A box or two of good defensive buckshot should not be unaffordable. Practice with the other stuff but keep the 8-pellet 00 loaded up when you get home, for bad days.