  We study the effects of a fourth generation t' quark in various extensions of
the standard model. In the Randall-Sundrum model, the decay t'--> t Z has a
large branching ratio that could be detected at the Large Hadron Collider
(LHC). We also look at the two-Higgs doublet models I, II and III, and note
that, in the latter, the branching ratio of t' --> t H, where H is a Higgs
scalar or pseudoscalar, is huge and we discuss detection at the LHC. A few
comments about the minimal supersymmetric standard model are also included.
