Pretty cool.
How much does it rain where he is?
If he is in high desert that 4000 gallon could go quick if he has a small prepper group to supply.
But if were he is he gets 3-4 major wet storms a year hen it might be good.
Certainly in an emergency (no long showers) 4000 gallons can last a small family (man, woman, 2 children) months if waste is closely controlled.