In the absence of M3x/4x, the circuit would have to be re-configured (to get the signal at the drain of M1/2 to appear as the outputs) but would give the output impedance as the drain/source impedance of M1/2 and the gain as the Gm
(M1/2) X the impedance of M3/4 in parallel and M1/2. With M3x/4x in circuit the output impedance becomes 1/Gm of M3x/4x and the gain Gm (M1/2) X impedance of M3/4.
Frank