Cascode Current Mirror 2Vt + 2Von
The gate voltage of M2 is the same as M4, equals the voltage at Iref input.

M4 sits on top of M3 that has a gate voltage of Vt+Von.

So the total a the Iref input = M4(Vgs) + M3(Vgs)

As you stated the Vgs of all devices is Vt+Von as they are all passing the same current so substitute in the above

Ref input = Vt+Von + Vt+Von = 2Vt + 2Von.