$968$
4. A natural number greater than 0, if the sum of all its divisors equals twice itself, such a number is called a perfect number or a complete number. For example, the divisors of 6 are $1,2,3,6,1+2+3+6=12$, so 6 is the smallest perfect number. The question of whether there are infinitely many perfect numbers remains one of the challenges that have puzzled humans. Research on perfect numbers can start with calculating the sum of all divisors of a natural number, the sum of all divisors of 567 is -