Ha..you are also trying to test my limits huh?!
For 4 pt and still assuming elastic, one can work out to be
Stress=Moment/Elastic modulus (For rectangular section, elastic modulus=bd^2/6)
Moment = (P/2)*l/3 (assume total load on the beam is P)
i.e. Stress=Pl/(b*d*d)
Having, i did come across...