  We study the effects of a vector-like SU(2) quark doublet as a fourth
generation. In this model we examine the chiral structure and the FCNC in the
quark sector by using radiative B meson decays in the allowed region for
parameters from $R_b = {\Gamma_{qq}}/{\Gamma_{had.}}$. We compute the ratio $R
= {Br (b \to d \gamma)}/{Br (b \to s \gamma)}$ in the model which realizes a
different chiral structure as well as FCNC. The constraints has been extracted
from the experimental results of B meson decays, the $T_{new}$ parameter of
oblique corrections and $R_b$. Under the natural assumption that the violation
of the $V-A$ structure in the light-quark sector is small, we can determine the
allowed region for most of the mixings parameters and the vector-like quark
masses. We show that there will be significant deviations in $R$ from the SM
prediction due to the FCNC's and the violation of the $V-A$ structure.
