Hi,
The pole value is simply gm3/Cn2 radians per second. The zero frequency can (roughly) be computed by understanding that at the zero, no current flows out of the output node. Thus,
the transconductance current of M2 entering the output node + the transconductance current of M1 mirrored by M3-M4 = 0.
This means,
gm1,2*Vin/2 + gm1,2*(Vin/2)*(1/(gm3+s*Cn))*gm3 = 0
Hence,
gm3=-(gm3+s*Cn)
So,
s=-2*gm3/Cn
This value of s is the zero frequency, which clearly is twice the second non-dominant pole.