Let $f(x)$ be a polynomial such that
\[f(x^2 + 1) = x^4 + 4x^2.\]Find $f(x^2 - 1).$
Let $y = x^2 + 1.$  Then $x^2 = y - 1,$ and $x^4 = y^2 - 2y + 1,$ so
\[f(y) = (y^2 - 2y + 1) + 4(y - 1) = y^2 + 2y - 3.\]Hence,
\[f(x^2 - 1) = (x^2 - 1)^2 + 2(x^2 - 1) - 3 = \boxed{x^4 - 4}.\]