  If the fourth generation fermions exist, the new quarks could influence the
branching ratios of the decays of $B\to X_s \gamma$ and $B\to X_sl^+l^-$. We
obtain two solutions of the fourth generation CKM factor
$V^{*}_{t^{'}s}V_{t^{'}b}$ from the decay of $B\to X_s \gamma$. We use these
two solutions to calculate the new contributions of the fourth generation quark
to Wilson coefficients of the decay of $B\to X_sl^+l^-$. The branching ratio
and the forward-backward asymmetry of the decay of $B\to X_sl^+l^-$ in the two
cases are calculated. Our results are quite different from that of SM in one
case, almost same in another case. If Nature chooses the formmer, the $B$ meson
decays could provide a possible test of the forth generation existence.
