Could there be a Fourth Generation?
  We investigate the possibility of incorporating a chiral fourth generation into a GUT model. We find that in order to do so, precision fits to electroweak observables demand the introduction of light (< M_Z) supersymmetric particles. This also enables us to provide decay channels to the fourth-generation quarks. Perturbative consistency sets an upper bound on the coloured supersymmetric spectrum. The upper limit on the lightest Higgs boson is calculated and found to be above the present experimental lower limit. 
C. D. Froggatt and J. E. Dubicki (Department of Physics and Astronomy,   University of Glasgow)