Ох, нет, всё не верно.
(1/(1 + 4x)^(1/2))' = ((1 + 4x)^(-1/2))' = -1/2 * (1 + 4x)^(-1/2 - 1) * (1 + 4x)' = -1/2 * (1 + 4x)^(-3/2) * 4 = -2 * (1 + 4x)^(-3/2)