  We discuss the time-dependent CP asymmetry of decay $B \to \phi K_S$ in an
extension of the Standard Model with both two Higgs doublets and additional
fourth-generation quarks. We show that although the Standard Model with
two-Higgs-doublet and the Standard model with fourth generation quarks alone
are not likely to largely change the effective $\sin 2 \beta$ from the decay of
$B \to \phi K_S $, the model with both additional Higgs doublet and
fourth-generation quarks can easily account for the possible large negative
value of $\sin 2 \beta$ without conflicting with other experimental
constraints. In this model, additional large CP violating effects may arise
from the flavor changing Yukawa interactions between neutral Higgs bosons and
the heavy fourth generation down type quark, which can modify the QCD penguin
contributions. With the constraints obtained from $b \to s \bar{s} s$ processes
such as $B \to X_s \gamma$ and $\Delta m_{B_s^0}$, this model can lead to the
effective $\sin 2 \beta$ to be as large as $- 0.4$ in the CP asymmetry of $B
\to \phi K_S$.
