  In this paper, we prove a special case of Campana--Peternell's conjecture in
dimension 4. Specifically, we show that a projective smooth fourfold $X$ with
$c^2_1(X)\cdot c_2(X)\neq 0$ and strictly nef anti-canonical divisor $-K_X$ is
a Fano fourfold. To this aim, we completely solve the non-vanishing conjecture
for strictly nef anti-canonical divisors in dimension 4.
