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We revisit the stability and triviality bounds on the Higgs boson mass in thecontext of the Standard Model with three and four generations (SM3 and SM4,respectively). In light of the recent results from LHC the triviality bound inthe SM3 has now become obsolete, and the stability bound implies for a Higgsmass of e.g. mH=115 GeV the onset of new physics before 650 TeV, whereas thereare no limits for mH>133 GeV. For the SM4, the stability and triviality curvesintersect and bound a finite region. As a consequence, the fourth generationfermions place stringent theoretical limits on the Higgs mass, and there is amaximal scale beyond which the theory cannot be perturbatively valid. We findthat the Higgs mass cannot exceed 700 GeV for any values of the fourthgeneration fermion masses. Turning the argument around, the absence of a Higgssignal for mH<600 GeV excludes a fourth generation with quark masses below 300GeV and lepton masses below 350 GeV. In particular, the quark bounds also holdfor the small mixing scenarios for which the direct limits from Tevatron andLHC are not applicable, and the lepton bounds we obtain are stronger than thecollider limits. If a Higgs boson lighter than 700 GeV is not observed, afourth generation of chiral fermions with perturbative Yukawa couplings will beconclusively excluded for the full range of parameters.

Implications of the Stability and Triviality Bounds on the Standard Model with Three and Four Chiral Generations