  If the Higgs-like particle at 126 GeV is the standard model Higgs boson, then
SM4, the simplest four generation extension of the SM, is inconsistent with the
most recent LHC data. However, 4G variations (BSM4) are possible if the new
particle is not the SM Higgs boson and/or if other new quanta modify its
production and decay rates. Since LHC searches have pushed 4G quarks to high
mass and strong coupling where perturbation theory eventually fails, we examine
the leading nondecoupling EW (electroweak) corrections at two loop order to
estimate the domain of validity for perturbation theory. We find that the two
loop hypercharge correction, which has not been included in previous EW fits of
4G models, makes the largest quark sector contribution to the rho parameter,
much larger even than the nominally leading one loop term. Because it is large
and negative, it has a big effect on the EW fits. It does not invalidate
perturbation theory since it only first appears at two loop order and is large
because it does not vanish for equal quark doublet masses, unlike the one loop
term. We estimate that perturbation theory is useful for $m_Q\simeq 600$ GeV
but begins to become marginal for $m_Q\, \gtap\, 900$ GeV. The results apply
directly to BSM4 models that retain the SM Higgs sector but must be
re-evaluated for non-SM Higgs sectors.
