A function f : R → R satisfies the relation f(x + y) = f(x) · f(y), ∀x, y ∈ R and f(x) ≠ 0, ∀x ∈ R. If f is differentiable at x = 0 and f'(0) = 4 and f(6) = 3, then f'(6) is equal to