Ruling out a 4th generation using limits on hadron collider Higgs   signals
  We consider the impact of a 4th generation on Higgs to $\gamma\gamma$ and $WW,ZZ$ signals and demonstrate that the Tevatron and LHC have essentially eliminated the possibility of a 4th generation if the Higgs is SM-like and has mass below 200 GeV. We also show that the absence of enhanced Higgs signals in current data sets in the $\gamma\gamma$ and $WW,ZZ$ final states can strongly constrain the possibility of a 4th generation in two-Higgs-doublet models of type II, including the MSSM. 
John F. Gunion
hep-ph