#4 lead enough to kill a Fox?

[3" 870 Shell Shucker]

#4 Lead is probably enough for anything smaller than a deer.....and a deer had better not walk within 20 yards, if you're using a really tight choke. 

#4's are bad news if you have pattern density.  But sadly, I haven't ever had a combo that would get anywhere near 100 at 40 yards.