  We consider a scalar dark matter model, the SM4+D, consisting of the standard
model with four generations (SM4) and a real gauge-singlet scalar called
darkon, D, as the weakly interacting massive particle (WIMP) dark-matter (DM)
candidate. We explore constraints on the darkon sector of the SM4+D from WIMP
DM direct-search experiments, including CDMS II and CoGeNT, and from the decay
of a B meson into a kaon plus missing energy. We find that a sizable portion of
the darkon parameter space is still compatible with the experimental data.
Since the darkon-Higgs interaction may give rise to considerable enhancement of
the Higgs invisible decay mode, the existence of the darkon could lead to the
weakening or evasion of some of the restrictions on the Higgs mass in the
presence of fourth-generation quarks. In addition, it can affect the
flavor-changing decays of these new heavy quarks into a lighter quark and the
Higgs boson, as the Higgs may subsequently decay invisibly. Therefore we also
study these flavor-changing neutral transitions involving the darkon, as well
as the corresponding top-quark decay t -> cDD, some of which may be observable
at the Tevatron or LHC and thus provide additional tests for the SM4+D.
