Perturbativity and a Fourth Generation in the MSSM
  We study an extension of the MSSM with a fourth generation of chiral matter. With this extension no value of $\tb$ allows the theory to stay perturbative up to the GUT scale. We suggest one model with extra vector-like states at the TeV scale that allows perturbativity all the way up to the GUT scale. 
Zeke Murdock, Satyanarayan Nandi, Zurab Tavartkiladze