The picure below is a improved current mirror. There are two equal input currents Iin and an output Iout. M1 converts Iin into BIASN2 to bias transistor M4B and M3A converts Iin into BIASN to bias transistor M4A.
So what is the role of transistor M3B here? It is a cascoded transistor and by adding it, the output impedance increases. However, I don't see why we want to increase output impedance at this branch.