Bounds on the New Generations
  We consider the bounds for the values of higgs mass $m_H$ and the mass of the extra quarks and leptons $m_{extra}$ derived from the stability of electroweak vacuum and from the absence of Landau pole in Higgs potential. We find that in the case of the absence of new physics up to the GUT scale the bounds on masses of 4th generation are so strong that one can hope to discover it at LEP2. 
V.A. Novikov