If x²-1 is a factor of the polynomial, both x-1 and x+1 are factors of it.
According to the remainder theorem, if a binomial x-a is a factor of a polynomial p(x), then p(a)=0.
If x-1 and x+1 are factors of the polynomial p(x)=ax⁴+bx³+cx²+dx+e, then p(1)=0 and p(-1)=0.