differential amplifier help

Hi,
I am new and want to ask about differential amplifier.
Can you shed some light on the circuit below?
Why transistors M2 and M4 are added in the circuit?

I learnt some basic about differential amplifier but when I see this circuit, it is confusing me.
Why transistors M2 and M4 are added in the circuit?

It is cascode current source structure (M2,M1 is mirrored to M3,M4). It makes the current source have higher impedance.

Spoke too quickly, It is a mixer (multiplier). V1, V2 would be chopper for input signal Vb1.

hi, i am not sure what you meant. This is a differential amplifier i saw in an old thread in this forum.