Comment by nspaced

d=4 is not a valid RSA private key. When p and q are prime λ(n) = lcm(p − 1, q − 1) is always even. e must be coprime with λ(n) so it's odd. de ≡ 1 (mod λ(n)) which means that de is 1 + an even number, so d must also always be odd.