  We study the effects of a fourth generation t-prime quark in various
extensions of the standard model. In the Randall-Sundrum model, the decay
t-prime -> 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-prime -> t H, where
H is a Higgs scalar or pseudoscalar, is huge and we discuss detection at the
LHC.
