Fourth generation effects in B̄s → π−K+ decay

We study the effects of the fourth quark generation model in B?s ? ??K+ decay. Combining with the up-to-date experimental measurement for and Acpdir(B?s ? ??K+) by the CDF Collaboration, we derive the new bound of weak phase bd, which are 0? < bd < 44?, 321? < bd < 360? and 0? < bd < 26?, 342? < bd < 360? for mt' = 400 and 600 GeV respectively. In these regions, and Acpdir(B?s ? ??K+) consist with the current experimental data within errors.