  Inspired by the report of D0 Collaboration on the state $X(5568)$ with four
different flavors, a similar state, $ud\bar{s}\bar{b}$ is investigated in the
present work. The advantage of looking for this state over the state $X(5568)$
with quark contents, $bu\bar{d}\bar{s}$ or $bd\bar{u}\bar{s}$, is that the $BK$
threshold is 270 MeV higher than that of $B_s \pi$, and it allows a large mass
region for $ud\bar{s}\bar{b}$ to be stable. The chiral quark model and Gaussian
expansion method are employed to do the calculations of four-quark states
$ud\bar{s}\bar{b}$ with quantum numbers $IJ^{P}$($I=0,1;~ J=0,1,2;~P=+$). Two
structures, diquark-antidiquark and meson-meson, with all possible color
configurations are considered. The results indicate that energies of the
tetraquark with diquark-antiquark configuration are all higher than the
threshold of $BK$, but for the state of $IJ^P=00^+$ in the meson-meson
structure, the energies are below the corresponding thresholds, where the color
channel coupling plays an important role. The distances between two objects
(quark/antiquark) show that the state is a molecular one.
