So here's the circuit:
I think M2 is a common source amplifier and M4 a common gate amplifier, so they form a cascode amplifier where the gain is given by : Av = -gm2 * r0, r0 is the impedance at the node where the drain of the common gate amplifier is at (Vop).
r0 = ( [rds8 // (rds2a * (...
