  We investigate the mass differences $\Delta M_{B_{d,s}}$ in the mixing
$B^0_{d,s}-\bar B^0_{d,s}$ with a new up-like quark $t^{\prime}$ in the
sequential fourth generation model. We give the basic formulae for $\Delta
M_{B_{d,s}}$ in this model and obtain two kinds of numerical results of $\Delta
M_{B_{s}}$ which is the function of $m_{t^{\prime}}$. We find that one of our
results can satisfy the present experimental low bound of $\Delta M_{B_s}$. We
also get the constraints of the fourth generation CKM factor
$V^{*}_{t^{'}b}V_{t^{'}d}$, which is the function of $t^{'}$, from the
experimental measurements of $\Delta M_{B_d}$. Thus, $\Delta M_{B_{d,s}}$
provide a possible test of the fourth generation and give a signal of the new
physics.
