You would begin with a simplifying assumption, that
the circuit is operating at null (VIN+ == VIN-) and
then M3, M4 equally share I0 (hence each runs I0/2).
You would use whichever MOS equation you like (it
would probably be one of the simpler ones) to solve
for Vgs, and then the source (tail) voltage of the pair
is (Vin+ - Vgs).
Now, I see nothing here to tell me what {S0} is, so
its voltage / derivation is (at least) equally mysterious.