current mirror biasing cascode
With the gate of M1 held at 2VT+2OV (by M3 &amp; M4), you need VT+OV at its source (a VGS lower, or VT+OV lower), so M1's drain needs to be OV higher, or a total of VT+2OV.

M2 doesn't have to be at Vt+OV -- it could be just OV, but then (with the bias you have), you couldn't keep M1 in saturation.